Regioselectively substituted cellulose esters produced in a tetraalkylammonium alkylphosphate ionic liquid process and products produced therefrom

ABSTRACT

This invention relates a cellulose solution comprising cellulose and at least one tetraalkylammonium alkylphosphate and processes to produce the cellulose solution. Another aspect of this invention relates to shaped articles prepared from a cellulose solution comprising cellulose and at least one tetraalkylammonium alkylphosphate. Another embodiment of this invention relates to compositions comprising derivatives of cellulose prepared from a cellulose solution comprising at least one tetraalkylammonium alkylphosphate. Another embodiment of this invention relates to compositions comprising regioselectively substituted cellulose esters prepared from a cellulose solution comprising cellulose and at least one tetraalkylammonium alkylphosphate. In another embodiment of the invention, the cellulose esters of the present invention are used as protective and compensation films for liquid crystalline displays.

RELATED APPLICATIONS

This application is a continuation application of U.S. Non-Provisionalapplication Ser. No. 12/539,817 filed on Aug. 12, 2009, whichapplication claims the benefit of U.S. Provisional Application Ser. No.61/169,560 entitled “Tetraalkylammonium Alkylphosphates” filed on Apr.15, 2009, which is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention is relates to the field of cellulose chemistry,specifically to cellulose solutions, the production of cellulose esters,and products produced therefrom.

BACKGROUND OF THE INVENTION

Cellulose is a β-1,4-linked polymer of anhydroglucose. Cellulose istypically a high molecular weight, polydisperse polymer that isinsoluble in water and virtually all common organic solvents. The use ofunmodified cellulose in wood or cotton products such as housing orfabric is well known. Unmodified cellulose is also utilized in a varietyof other applications usually as a film, such as cellophane, as a fiber,such as viscose rayon, or as a powder, such as microcrystallinecellulose used in pharmaceutical applications. Modified cellulose suchas cellulose esters are also widely utilized in a wide variety ofcommercial applications [Prog. Polym. Sci. 2001, 26, 1605-1688].Cellulose esters are generally prepared by first converting cellulose toa cellulose triester before hydrolyzing the cellulose triester in anacidic aqueous media to the desired degree of substitution (DS, thenumber of substituents per anhydroglucose monomer). Hydrolysis ofcellulose triacetate under these conditions yields a random copolymerthat can consist of 8 different monomers depending upon the final DS[Macromolecules 1991, 24, 3050].

The dissolution of cellulose in ionic liquids is known. Those skilled inthe art will recognize that the maximum amount of cellulose dissolved atany given temperature in a particular ionic liquid will depend upon thedegree of polymerization (DP) of the initial cellulose and the extent ofdegradation of the cellulose during the dissolution process.

In the broadest sense, an ionic liquid (IL) is simply any liquidcontaining only ions. Hence, molten salts such as NaCl which melts attemperatures greater than 800° C. could be classified as ionic liquids.From a practical point of view, the term ionic liquid is now used fororganic salts that melt below approximately 100° C.

Although the cations of ionic liquids are structurally diverse, theygenerally contain nitrogen or phosphorus that can be converted to aquaternary ammonium or phosphonium. In general, the most useful ionicliquids contain nitrogen that is part of a ring structure. Examples ofthese cations include pyridinum, pyridazinium, pyrimidinium, pyrazinium,imidazolium, pyrazolium, oxazolium, triazolium, thiazolium,piperidinium, pyrrolidinium, quinolinium, and isoquinolinium.

The anions of ionic liquids can also be structurally diverse. The anioncan be either inorganic or organic and they can have a significantimpact on the solubility of the ionic liquids in different media. Forexample, ionic liquids containing hydrophobic anions such ashexafluorophosphates or triflimides have very low solubilities in waterwhile ionic liquids containing hydrophilic anions such chloride oracetate are completely miscible in water.

The names of ionic liquids are generally abbreviated. Alkyl cations areoften named by the letters of the alkyl substituents and the cationwhich are given within a set of brackets followed by the abbreviationfor the anion. Although not expressively written, it is understood thatthe cation has a positive charge and the anion has a negative charge.For example, [BMIm]OAc means 1-butyl-3-methylimidazolium acetate and[AMIm]Cl means 1-allyl-3-methylimidazolium chloride.

In the case in which nitrogen is part of a cyclic cation (e.g.imidazolium), these ionic liquids typically have lower melting points,are less hydroscopic, and are more stable that the correspondingammonium or phosphonium containing ionic liquids. In terms of cellulosedissolution and esterification, ionic liquids containing imidazolium asthe cation are generally preferred. With these ionic liquids, it ispossible to achieve concentrated cellulose solutions (ca. 20 wt % in[EMIm]OAc) from which a variety of cellulose esters can be prepared(U.S. Pat. No. 6,824,599, US application 20080194807, US application20080194808, and US application 20080194834).

The most commonly utilized non-cyclic ammonium based ionic liquids aretetraalkylammonium halides. This class of ionic liquids has limitedutility in cellulose dissolution and esterification. Only twotetraalkylammonium halides, tetraethylammonium chloride (U.S. Pat. No.4,597,798) and tetrabutylammonium fluoride hydrate (Macromol. Biosci.2007, 7, 307-314), have been shown to solubilize cellulose at asignificant level (ca. 6 wt % cellulose). In both cases, significantconcentrations of cosolvents (30-90 wt %) such as DMSO or DMF wererequired for cellulose solubilization. Due to low concentrations ofcellulose in the ionic liquids, the difficulty of removing water fromtheses ionic liquids, the instability of these ionic liquids, as well asthe corrosive and toxic nature of these ionic liquids, thesetetraalkylammonium halides are not practical media for making cellulosederivatives.

SUMMARY OF THE INVENTION

One embodiment of this invention relates to the dissolution of cellulosein one or more tetraalkylammonium alkylphosphates. The invention furtherrelates to the dissolution of cellulose in a mixture comprising one ormore tetraalkylammonium alkylphosphates and one or more aprotic solventsat a contact time and temperature sufficient to dissolve cellulose. Theinvention yet further relates to the dissolution of cellulose in amixture comprising one or more tetraalkylammonium alkylphosphates andone or more ionic liquids wherein the cation is imidazolium. Theinvention still further relates to the dissolution of cellulose in amixture comprising one or more tetraalkylammonium alkylphosphates andone or more acids wherein the acids do not combine with the cellulose ata contact time and temperature sufficient to dissolve cellulose. Anotherembodiment of this invention relates to shaped articles prepared from acellulose solution comprising at least one tetraalkylammoniumalkylphosphate or mixtures thereof. Another embodiment of this inventionrelates to compositions comprising derivatives of cellulose preparedfrom a cellulose solution comprising at least one tetraalkylammoniumalkylphosphate or mixtures thereof. Another embodiment of this inventionrelates to compositions comprising regioselectively substitutedcellulose esters prepared from a cellulose solution comprising at leastone tetraalkylammonium alkylphosphate or mixtures thereof. In yetanother embodiment of the invention, the cellulose esters of the presentinvention are used as protective and compensation films for liquidcrystalline displays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the dissolution of 12.5 wt % cellulose intributylmethylammonium dimethyl phosphate ([TBMA]DMP). Celluloseabsorbance was measured at 1000 cm⁻¹.

FIG. 2 shows modeled wt % cellulose and the experimental absorbancevalues for 10 wt % cellulose dissolved in [TBMA]DMP.

FIG. 3 shows a plot of absorbance for infrared bands at 1825 cm⁻¹(acetic anhydride and 1220 cm⁻¹ (acetate ester and acetic acid) versuscontact time during esterification (3 eq acetic anhydride) of cellulosedissolved in [TBMA]DMP.

FIG. 4 shows a plot of absorbance for infrared bands at 1825 cm⁻¹(acetic anhydride and 1220 cm⁻¹ (acetate ester and acetic acid) versuscontact time during esterification of the cellulose dissolved in[BMIm]DMP.

FIG. 5 compares the absorbance for infrared bands at 1825 cm⁻¹ (aceticanhydride and 1220 cm⁻¹ (acetate ester and acetic acid) versus contacttime during esterification of the cellulose dissolved in [BMIm]DMP inthe absence and in the presence of MSA.

FIG. 6 shows a plot of absorbance for infrared bands at 1825 cm⁻¹(acetic anhydride) and 1724 cm⁻¹ (acetic acid) versus contact timeduring esterification of cellulose dissolved in [TBMA]DMP.

FIG. 7 shows the contact period involving the addition of 0.5 eq Ac2O at100° C. The x axis has been shifted so that each reaction begins at thesame point of anhydride addition (15 min).

FIG. 8 shows the contact period involving the addition of 2.5 eq Ac₂O at80° C. The x axis has been shifted so that each reaction begins at thesame point of anhydride addition (72 min).

FIG. 9 shows a plot of DS versus time for the time period involvingaddition of 2.5 eq Ac₂O in the absence and presence of acid at 80° C.

FIG. 10 shows a plot of absorbance for infrared bands at 1815 cm-1(anhydride), 1732 cm-1 (acid), and 1226 cm-1 (ester+acid) versus contacttime during esterification of cellulose dissolved in [TBMA]DMP.

FIG. 11 compares a plot of absorbance for infrared bands at 1815 cm⁻¹and 1732 cm⁻¹ versus contact time during the esterification of cellulosedissolved in [TBMA]DMP when Ac₂O/Pr₂O or Ac₂O/Pr₂O+MSA are added at 100°C.

FIG. 12 compares a plot of absorbance for infrared bands at 1815 cm-1and 1732 cm-1 versus contact time during the esterification of cellulosedissolved in [TBMA]DMP when Ac2O/Pr2O or Ac2O/Pr2O+MSA are added at 60°C.

FIG. 13 shows a plot of DS versus time for the time period involvingaddition of 1.0 eq Ac2O and 1.0 eq Pr2O in the absence and presence ofMSA at 60° C.

FIG. 14 shows a plot of absorbance for infrared bands at 1825 cm-1(acetic anhydride and 1724 cm-1 (acetic acid) versus contact time duringesterification of cellulose dissolved in 75/25 wt/wt [TBMA]DMP/DMFmixture.

FIG. 15 shows the relationship between R_(th) and the total degree ofsubstitution for the regioselectively substituted cellulose esters ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood more readily by reference to thefollowing detailed description of the invention and the examplesprovided therein. It is to be understood that this invention is notlimited to the specific methods, formulations, and conditions described,as such may vary. It is also to be understood that the terminology usedherein is for the purpose of describing particular aspects of theinvention only and is not intended to be limiting.

In this specification and in the claims that follow, reference will bemade to a number of terms, which shall be defined to have the followingmeanings.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

Values may be expressed as “about” or “approximately” a given number.Similarly, ranges may be expressed herein as from “about” one particularvalue and/or to “about” or another particular value. When such a rangeis expressed, another aspect includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another aspect.

One embodiment of this invention relates to the dissolution of cellulosein one or more tetraalkylammonium alkylphosphates at a contact time andtemperature sufficient to dissolve cellulose. The invention furtherrelates to the dissolution of cellulose in a mixture comprising one ormore tetraalkylammonium alkylphosphates and one or more aprotic solventsat a contact time and temperature sufficient to dissolve cellulose. Theinvention yet further relates to the dissolution of cellulose in amixture comprising one or more tetraalkylammonium alkylphosphates andone or more ionic liquids wherein the cation is imidazolium at a contacttime and temperature sufficient to dissolve cellulose. The inventionstill further relates to the dissolution of cellulose in a mixturecomprising one or more tetraalkylammonium alkylphosphates and one ormore acids wherein the acids do not combine with the cellulose at acontact time and temperature sufficient to dissolve cellulose. Anotherembodiment of this invention relates to shaped articles prepared from acellulose solution comprising at least one tetraalkylammoniumalkylphosphate.

The cellulose suitable for the present invention can be any celluloseknown in the art suitable for the production of cellulose esters. In oneembodiment, the cellulose can be obtained from soft or hard woods aswood pulps or from annual plants such as cotton or corn. The cellulosecan be a β-1,4-linked polymer comprising a plurality of anhydroglucosemonomer units. The cellulose suitable for use in the present inventioncan generally comprise the following structure:

The cellulose can have an α-cellulose content of at least 90%, and morepreferably, an α-cellulose content of at least 95%. The cellulose canhave a degree of polymerization (DP) of at least 10, at least 250, atleast 1000 or at least 5,000. As used herein, the term “degree ofpolymerization,” when referring to cellulose and/or cellulose esters,shall denote the average number of anhydroglucose monomer units percellulose polymer chain. Furthermore, the cellulose can have a weightaverage molecular weight in the range of from about 1,500 to about850,000, in the range of from about 40,000 to about 200,000, or in therange of from 55,000 to about 160,000. Additionally, the cellulosesuitable for use in the present invention can be in the form of a sheet,a hammer milled sheet, fiber, or powder. In one embodiment, thecellulose can be a powder having an average particle size of less thanabout 500 micrometers (“μm”), less than about 400 μm, or less than 300μm.

The tetraalkylammonium alkylphosphates suitable for the presentinvention correspond to the structures:

wherein R1, R2, R3, and R4 are independently a C₁-C₅ straight chain orbranched alkyl group or a C₂-C₂₀ alkoxy group, and R5 and R6 areindependently a hydrido, a C₁-C₅ straight chain or branched alkyl group,or a C₂-C₂₀ alkoxy group. In one embodiment of the invention, R1 ismethyl or ethyl and R2, R3, and R4 are independently methyl, ethyl,propyl, butyl, isobutyl, pentyl, ethanol, ethoxyethanol wherein R1, R2,R3, and R4 are not identical, and R5 and R6 are methyl, ethyl, propyl,or butyl. In another embodiment, R1 is methyl, and R2, R3, and R4 arepropyl or butyl, and R5 and R6 are methyl or ethyl. Examples of suitabletetraalkylammonium alkylphosphates include, but are not limited to,tributylmethylammonium dimethylphosphate ([TBMA]DMP),tributylethylammonium diethylphosphate ([TBEA]DEP),tripropylmethylammonium dimethylphosphate ([TPMA]DMP),tripropylethylammonium diethylphosphate ([TPEA]DEP). The preparations ofthese tetraalkylammonium alkylphosphates for use as lubricants andantistatic agents have been disclosed in U.S. Pat. No. 2,563,506 (1951),which is herein incorporated by reference to the extent it does notcontradict the statements herein.

In one embodiment, the ratio of cation:anion of the tetraalkylammoniumalkylphosphate is from about 1:1 to about 1:10. In another embodiment,the ratio of cation:anion is about 1:1. In yet another embodiment, theratio of cation:anion is about 1:2.

One or more cosolvents mixed with tetraalkylammonium alkylphosphates canbe useful in preparing solutions of cellulose. Cosolvents include, butare not limited to, aprotic solvents, protic solvents, acids, and ionicliquids other than tetraalkylammonium alkylphosphates. For example,aprotic solvents are useful in the present invention. In the presentinvention, aprotic solvents are those which do not contain hydrogenattached to oxygen, nitrogen, or sulfur that can dissociate and have adielectric constant greater than about 30. Further, the suitable aproticsolvents do not have an acidic proton that can be removed by base duringcellulose dissolution or esterification. When aprotic solvents areutilized with tetraalkylammonium alkylphosphates, higher celluloseconcentrations with lower cellulose solution viscosities can be achievedallowing for reduced contact temperatures. Further, the cellulose esterproducts often have better solubility when utilizing aprotic solventsthan they do in tetraalkylammonium alkylphosphates alone, which is oftenimportant when making selected cellulose ester compositions. Examples ofsuitable aprotic solvents include hexamethylphosphoramide,N-methylpyrrolidone, nitromethane, dimethylformamide, dimethylacetamide,acetonitrile, sulfolane, dimethyl sulfoxide, and the like. In oneembodiment, aprotic solvents include, but are not limited to,N-methylpyrrolidone (NMP) and dimethylformamide (DMF). The aproticsolvents may be present in an amount from about 0.1 to about 99 wt %based on total weight of the cosolvents and tetraalkylammoniumalkylphosphates. In another embodiment of the invention, the amount ofcosolvents is from about 5 to about 90 wt % or from about 10 to about 25wt % based on total weight of the cosolvents and tetraalkylammoniumalkylphosphates.

Certain protic solvents are also useful as cosolvents in the presentinvention. For the purpose of this invention, protic solvents are thosewhich contain hydrogen attached to oxygen, nitrogen, or sulfur that candissociate and have a dielectric constant less than about 10. Examplesof suitable protic solvents include aliphatic carboxylic acids such asacetic acid, propionic acid, butyric acid, isobutyric acid and aminessuch as diethyl amine, butyl amine, dibutyl amine, propyl amine,dipropyl amine. In another embodiment of the invention, protic solventsinclude acetic acid, propionic acid, butyric acid and combinationsthereof. The protic solvents may be present in an amount from about 0.1to about 25 wt % based on total weight of the cosolvents andtetraalkylammonium alkylphosphates. In another embodiment, the amount ofcosolvents is from about 1 to about 15 wt % or from about 3 to about 10wt % based on total weight of the cosolvents and tetraalkylammoniumalkylphosphates.

In another embodiment of the invention, ionic liquids, other thantetraalkylammonium alkylphosphates, can be utilized as a cosolvent. Theionic liquid can be any known in the art capable of assisting indissolving the cellulose in the tetraalkylammonium alkylphosphate.Suitable examples of such ionic liquids are disclosed in U.S. patentapplication entitled “Cellulose Esters and Their Production InCarboxylated Ionic Liquids” filed on Feb. 13, 2008 and having Ser. No.12/030,387 and in U.S. patent application entitled “Cellulose Esters andTheir Production in Halogenated Ionic Liquids filed on Aug. 11, 2008 andhaving Ser. No. 12/189,415; both of which are herein incorporated byreference to the extent they do not contradict the statements herein.

In one embodiment of the invention, the ionic liquids useful ascosolvents in the present invention are alkyl or alkenyl substitutedimidazolium salts corresponding to the structure:

wherein R1 and R3 are independently a C₁-C₈ alkyl group, a C2-C8 alkenylgroup, or a C₁-C₈ alkoxyalkyl group, and R2, R4, and R5 areindependently a hydrido, a C₁-C₈ alkyl group, a C₂-C₈ alkenyl group, aC₁-C₈ alkoxyalkyl group, or a C₁-C₈ alkoxy group. The anions (X⁻) arechloride, C₁-C₂₀ straight chain or branched carboxylate or substitutedcarboxylate, or alkylphosphates. Examples of carboxylate anions includeformate, acetate, propionate, butyrate, valerate, hexanoate, lactate,oxalate, or chloro-, bromo-, fluoro-substituted acetate, propionate, orbutyrate and the like. Examples of dialkylphosphates includedimethylphosphate, diethylphosphate, dipropylphosphate, ordibutylphosphate and the like. Examples of these types of imidazoliumsalts for cellulose dissolution can be found in US applications20080194807, 20080194808, 20080194834; Green Chemistry 2008, 10, 44-46;Green Chemistry 2007, 9, 233-242. The imidazolium based ionic liquidsmay be present in an amount from about 99 wt % to about 1 wt % based onthe total weight of liquid components used to dissolve cellulose. Inanother embodiment, the amount of imidazolium based ionic liquid is fromabout 75 wt % to about 2 wt % or about 20 wt % to about 5 wt % based onthe total weight of liquid used to dissolve cellulose

Acids that may be mixed with one or more tetraalkylammoniumalkylphosphates as a cosolvent are those which do not combine with thecellulose during cellulose dissolution or esterification. Examples ofsuitable acids include, but are not limited to, alkyl sulfonic acids,such as, methane sulfonic acid and aryl sulfonic acids, such as,p-toluene sulfonic acid. The acids may be present in an amount fromabout 0.01 to about 10 wt % based on total weight of the acid andtetraalkylammonium alkylphosphate, and optionally, other mixturecomponents. In another embodiment of the invention, the amount of acidis from about 0.1 to about 7 wt % and or from about 1 to about 5 wt %based on total weight of the acid plus tetraalkylammonium alkylphosphateand, optionally, other mixture components. In one aspect of thisembodiment, the acid does not significantly alter the molecular weightof the cellulose.

In another aspect of this embodiment, at least one acid can be mixed inafter the cellulose is dissolved in with one or more tetraalkylammoniumalkylphosphates. In another aspect of this embodiment, the acid can bemixed with the tetraalkylammonium alkylphosphates prior to cellulosedissolution. In another embodiment, the acid can be added as a mixturewith a portion of the acylating reagents. Surprisingly, it has beenfound that an acid can slow reaction rates and does not lead tosignificant molecular weight reduction. Moreover, the color of thecellulose ester product is improved relative to when the acid is absent.Hence, in one aspect of this embodiment, the acid can lead to a slowerreaction rate relative to when no acid is present. In another aspect,the acid does not significantly alter the molecular weight of thecellulose. In yet another aspect, the acid provides improved celluloseester color relative to when no acid is present. The precise reactionrate change, the product molecular weights, and the improvement inproduct color depends upon a number of factors such as selection of theacid, acid concentration, contact temperature, and contact time.

When dissolving cellulose in the present invention to produce acellulose solution, the contact temperature and time is that which issufficient to obtain a homogeneous mixture of the cellulose in thetetraalkylammonium alkylphosphate. In one embodiment of the invention,the contact temperature is from about 20° C. to about 150° C. or fromabout 50° C. to about 120° C. In one embodiment of the invention, thecontact time is from about 5 min to about 24 hours or from about 30 minto about 3 hours. Those skilled in the art will understand that the rateof dissolution is dependent upon temperature and how well the celluloseis dispersed in the tetraalkylammonium alkylphosphate. The amount ofcellulose that can be dissolved in the tetraalkylammonium alkylphosphateof the present invention depends upon the particular tetraalkylammoniumalkylphosphate used to dissolve the cellulose and the DP of thecellulose. In one embodiment, the concentration of cellulose in thecellulose solution is from about 1 wt % to about 40 wt % based on thetotal weight of the cellulose solution or from about 7 wt % to about 20wt %.

In one embodiment of the invention, it is not necessary for the liquidin which at least one component is tetraalkylammonium alkylphosphate tobe free of H₂O, nitrogen containing bases, or alcohol acid prior todissolution of the cellulose. Therefore, the cellulose solution cancomprise cellulose and tetraalkylammonium alkylphosphate, and at leastone component selected from the group consisting of water, nitrogencontaining bases, and alcohol. In one embodiment, the tetraalkylammoniumalkylphosphates contains less than about 20 wt % water, nitrogencontaining base, and/or alcohol based on the total weight of liquidscontained in the cellulose solution. In another embodiment, thetetraalkylammonium alkylphosphate contains less than about 5 wt % water,base, and/or alcohol or less than about 2 wt % water, base, and/oralcohol based on the total weight of liquids contained in the cellulosesolution.

In another embodiment of the invention, a variety of shaped articles canbe prepared from a cellulose solution comprising at least onetetraalkylammonium alkylphosphate or mixtures thereof. These shapedforms include powders, films, fibers, tubes, and the like. Typically,the cellulose solution is formed into the desired shape and thenimmediately contacted with a non-solvent that will cause regeneration orprecipitation of the cellulose but is miscible with thetetraalkylammonium alkylphosphate. Examples of such non-solventsinclude, but are not limited to, water and alcohols, such as, methanol,ethanol, n-propanol, iso-propanol, and the like. As an example of thisprocess, the cellulose solution can be extruded through a die whichimparts a certain shape, and then, the cellulose solution is contactedwith the non-solvent thereby forming a fiber. In the case of electrospinning, it is possible to form nano fibers. Another example is castingof a thin film onto a surface which is then contacted with thenon-solvent thereby forming a solid film.

Another embodiment of this invention relates to compositions comprisingcellulose derivatives prepared from a cellulose solution comprisingcellulose and at least one tetraalkylammonium alkylphosphate or mixturesthereof. The cellulose derivatives may be cellulose esters, celluloseethers, or mixed cellulose ester-ethers.

Cellulose esters can be prepared by contacting the cellulose solutionwith one or more C₁-C₂₀ acylating reagents at a contact temperature andcontact time sufficient to provide a cellulose ester with the desireddegree of substitution (DS) and degree of polymerization (DP). Thecellulose esters thus prepared generally comprise the followingstructure:

wherein R₂, R₃, R₆ are hydrogen, with the proviso that R₂, R₃, R₆ arenot hydrogen simultaneously, or C₁-C₂₀ straight- or branched-chain alkylor aryl groups bound to the cellulose via an ester linkage.

The cellulose esters prepared by the methods of the present inventionhave a DS from about 0.1 to about 3.5, from about 0.1 to about 3.08,from about 0.1 to about 3.0, from about 1.8 to about 2.9, or from about2.0 to about 2.6. The DP of the cellulose esters prepared by the methodsof the present invention will be at least 5 or at least 10. In anotherembodiment, the DP of the cellulose esters is at least 50 or at least100, or at least 250. In yet another embodiment, the DP of the celluloseesters is from about 5 to about 1000, from about 10 to about 250 or fromabout 10 to about 50.

The acylating agents can be any known in the art for acylating celluloseto produce cellulose esters. In one embodiment of the invention, theacylating reagent is one or more C₁-C₂₀ straight- or branched-chainalkyl or aryl carboxylic anhydrides, carboxylic acid halides, diketene,or acetoacetic acid esters. Examples of carboxylic anhydrides include,but are not limited to, acetic anhydride, propionic anhydride, butyricanhydride, isobutyric anhydride, valeric anhydride, hexanoic anhydride,2-ethylhexanoic anhydride, nonanoic anhydride, lauric anhydride,palmitic anhydride, stearic anhydride, benzoic anhydride, substitutedbenzoic anhydrides, phthalic anhydride, and isophthalic anhydride.Examples of carboxylic acid halides include, but are not limited to,acetyl, propionyl, butyryl, hexanoyl, 2-ethylhexanoyl, lauroyl,palmitoyl, benzoyl, substituted benzoyl, and stearoyl halides. Examplesof acetoacetic acid esters include, but are not limited to, methylacetoacetate, ethyl acetoacetate, propyl acetoacetate, butylacetoacetate, and tert-butyl acetoacetate. In one embodiment of theinvention, the acylating reagent is at least one C₂-C₉ straight- orbranched-chain alkyl carboxylic anhydrides selected from the groupconsisting of acetic anhydride, propionic anhydride, butyric anhydride,2-ethylhexanoic anhydride, nonanoic anhydride, and stearic anhydride.The acylating reagents can be added after the cellulose has beendissolved in the tetraalkylammonium alkylphosphate. If so desired, theacylating reagent can be added to the tetraalkylammonium alkylphosphateprior to dissolving the cellulose in the tetraalkylammoniumalkylphosphate. In another embodiment, the tetraalkylammoniumalkylphosphate and the acylating reagent can be added simultaneously tothe cellulose to produce the cellulose solution.

In the esterification of cellulose dissolved in tetraalkylammoniumalkylphosphates, the contact temperature is that which is sufficient toproduce the desired cellulose ester. In one embodiment, the contacttemperature is from about 20° C. to about 140° C. In another embodiment,the contact temperature is from about 50° C. to about 100° C. or fromabout 60° C. to about 80° C.

In the esterification of cellulose dissolved in tetraalkylammoniumalkylphosphates, the contact time is that which is sufficient to producethe desired cellulose ester. In one embodiment of the invention, thecontact time is from about 1 min to about 48 hours. In anotherembodiment, the contact time is from about 10 min to about 24 hours, orfrom about 30 min to about 5 hours.

It is important to understand that in the present invention, little orno cellulose phosphate ester is formed during the celluloseesterification. That is, the tetraalkylammonium alkylphosphate is notacting as a phosphate donor. This is in contrast to ionic liquids, suchas, 1,3-dialkylimidazolium carboxylates wherein at least one of the acylgroups of the cellulose ester is donated by the ionic liquid, which isdescribed in U.S. patent application entitled “Cellulose Esters andTheir Production In Carboxylated Ionic Liquids” filed on Feb. 13, 2008and having Ser. No. 12/030,387, which has previously been incorporatedby reference to the extent it does not contradict the statements herein.That is, the ionic liquid acts as an acylating reagent. The celluloseesters of the present invention can be prepared by a process comprising:

-   -   a) contacting cellulose with at least one tetraalkylammonium        alkylphosphate to form a cellulose solution;    -   b) contacting the cellulose solution with at least one acylating        reagent at a contact temperature and contact time sufficient to        produce an acylated cellulose solution comprising at least one        cellulose ester;    -   c) contacting the acylated cellulose solution with at least one        non-solvent to cause the cellulose ester to precipitate to        produce a cellulose ester slurry comprising precipitated        cellulose ester and the tetraalkylammonium alkylphosphate;    -   d) separating at least a portion of the precipitated cellulose        ester from the cellulose ester slurry to produce a recovered        cellulose ester and precipitation liquids comprising the        tetraalkylammonium alkylphosphate.

In another embodiment of this invention, the process of producingcellulose esters further comprises washing the recovered cellulose esterwith a wash liquid to produce a washed cellulose ester.

In another embodiment of the invention, the process of producingcellulose esters further comprises drying the washed cellulose ester toproduce a dried cellulose ester product.

In another embodiment of the invention, the process of producingcellulose esters further comprises separating the tetraalkylammoniumalkylphosphate from the precipitation liquids to produce a recoveredtetraalkylammonium alkylphosphate.

In another embodiment of the invention, the process of producingcellulose esters further comprises recycling the recoveredtetraalkylammonium alkylphosphate to dissolve cellulose to produce acellulose solution.

In the process of the present invention, when contacting the one or moreacylating reagents with the cellulose solution the amount of acylatingreagent that is added and the order of which the acylating reagent isadded can significantly influence factors such as solution viscosity,product quality, and relative degree of substitution (RDS).

In one aspect of the invention, the one or more acylating reagents areadded in a single addition. In this aspect, about 0.1 equivalents toabout 20 equivalents of acylating reagent are added during one additionperiod wherein equivalents is the number of moles of acylating reagentper mole of anhydroglucose. In another embodiment about 0.5 equivalentsto about 5 equivalents of acylating reagent are added during oneaddition period. Most preferred is when about 1 equivalents to about 3equivalents acylating reagent is added during one addition period.

In another aspect of the invention, the acylating reagent addition isstaged meaning that the acylating reagent is added consecutively. Inthis aspect, a total of 0.5 equivalent to about 20 equivalents ofacylating reagent is added to the cellulose solution or a total of about1.5 equivalents to about 5 equivalents acylating reagent is added.However, in a staged addition, about 0.1 equivalents to about 2equivalents acylating reagent is added during one addition period, andthe remaining acylating reagent is added in one or more differentaddition periods. In another embodiment, about 0.5 equivalents to about1 equivalent of acylating reagent are added during one addition period,and the remaining acylating reagent is added in one or more differentaddition periods. One benefit of this aspect of the invention is thatthe initial acylating reagent addition can lead to a reduction insolution viscosity of the contact mixture. Reduction in solutionviscosity allows easier movement from one vessel to another and also canallow reduction of contact temperature during subsequent acylatingreagent additions which can impact product quality. Another benefit ofthis aspect of the invention relates to when two or more acylatingreagents are added in a staged addition. In this aspect, one acylatingreagent can be added and allowed to react during the 1^(st) stage then asecond acylating reagent can be added and allowed to react during the2^(nd) stage thereby leading to novel cellulose esters with uniquesubstitution patterns.

In the present invention, we found that when adding one or moreacylating reagents, the C₆ position of cellulose was acylated muchfaster than C₂ and C₃. Consequentially, the C₆/C₃ and C₆/C₂ RDS ratiosare greater than 1 which is characteristic of a regioselectivelysubstituted cellulose ester. The degree of regioselectivity depends uponat least one of the following factors: type of acyl substituent, contacttemperature, ionic liquid interaction, equivalents of acyl reagent,order of additions, and the like. Typically, the larger the number ofcarbon atoms in the acyl substituent, the C₆ position of the celluloseis acylated preferentially over the C₂ and C₃ position. In addition, asthe contact temperature is lowered in the esterification, the C₆position of the cellulose can be acylated preferentially over the C₂ orC₃ position. As mentioned previously, the type of ionic liquid and itsinteraction with cellulose in the process can affect theregioselectivity of the cellulose ester. For example, when carboxylatedionic liquids are utilized, a regioselectively substituted celluloseester is produced where the RDS is C₆>C₂>C₃. When thetretraalkylammonium dialkylphosphates of the present invention areutilized, a regioselectively substituted cellulose ester is producedwhere the RDS is C₆>C₃>C₂. This is significant in that regioselectiveplacement of substituents in a cellulose ester leads to regioselectivelysubstituted cellulose esters with different physical properties relativeto conventional cellulose esters.

In one embodiment of this invention, no protective groups are utilizedto prevent reaction of the cellulose with the acylating reagent.

In one embodiment of the present invention, the ring RDS ratio for C₆/C₃or C₆/C₂ is at least 1.05. In another embodiment, the ring RDS ratio forC₆/C₃ or C₆/C₂ is at least 1.1. Another embodiment of the presentinvention is when the ring RDS ratio for C₆/C₃ or C₆/C₂ is at least 1.3.

In another embodiment of the present invention, the product of the ringRDS ratio for C₆/C₃ or C₆/C₂ times the total DS [(C₆/C₃)*DS or(C₆/C₂)*DS] is at least 2.9. In another embodiment, the product of thering RDS ratio for C₆/C₃ or C₆/C₂ times the total DS is at least 3.0. Inanother embodiment, the product of the ring RDS ratio for C₆/C₃ or C₆/C₂times the total DS is at least 3.2.

In another embodiment of the present invention, the ring RDS ratio forC₆/C₃ or C₆/C₂ is at least 1.05, and the product of the ring RDS ratiofor C₆/C₃ or C₆/C₂ times the total DS is at least 2.9. In anotherembodiment, the ring RDS ratio for C₆/C₃ or C₆/C₂ is at least 1.1, andthe product of the ring RDS ratio for C₆/C₃ or C₆/C₂ times the total DSis at least 3.0. In yet another embodiment, the ring RDS ratio for C₆/C₃or C₆/C₂ is at least 1.3, and the product of the ring RDS ratio forC₆/C₃ or C₆/C₂ times the total DS is at least 3.2.

As noted previously, when 2 or more acyl substituents are present inmore equal amounts, it is sometimes desirable to integrate the carbonylcarbons in order to determine the RDS of each substituent independently.Hence, in one embodiment of the present invention, the carbonyl RDSratio of at least one acyl substituent for C₆/C₃ or C₆/C₂ is at least1.3. In another embodiment, the carbonyl RDS ratio of at least one acylsubstituent for C₆/C₃ or C₆/C₂ is at least 1.5. In another embodiment,the carbonyl RDS ratio of at least one acyl substituent for C₆/C₃ orC₆/C₂ is at least 1.7.

In another embodiment of the present invention, the product of thecarbonyl RDS ratio of at least one acyl substituent for C₆/C₃ or C₆/C₂times the DS of the acyl substituent [(C₆/C₃)*DS_(acyl) or(C₆/C₂)*DS_(acyl)] is at least 2.3. In another embodiment, the productof the carbonyl RDS ratio for C₆/C₃ or C₆/C₂ times the DS of the acylsubstituent is at least 2.5. In another embodiment, the product of thecarbonyl RDS ratio for C₆/C₃ or C₆/C₂ times the DS of the acylsubstituent is at least 2.7. In another embodiment of the presentinvention, the carbonyl RDS ratio of at least one acyl substituent forC₆/C₃ or C₆/C₂ is at least 1.3, and the product of the carbonyl RDSratio for C₆/C₃ or C₆/C₂ times the acyl DS is at least 2.3. In anotherembodiment, the carbonyl RDS ratio for C₆/C₃ or C₆/C₂ is at least 1.5,and the product of the carbonyl RDS ratio for C₆/C₃ or C₆/C₂ times theacyl DS is at least 2.5. In yet another embodiment, the carbonyl RDSratio for C₆/C₃ or C₆/C₂ is at least 1.7 and the product of the carbonylRDS ratio for C₆/C₃ or C₆/C₂ times the acyl DS is at least 2.7.

Surprisingly, in one embodiment of the invention, staged additions ofthe acylating reagent gave a relative degree of substitution (RDS)different from that obtained in the mixed addition of acylating reagent.Moreover, both the staged and mixed additions of the present inventionprovide a different RDS relative to other means known in the prior artfor making mixed cellulose esters which generally provide celluloseesters with a RDS at C₆, C₃, and C₂ of about 1:1:1. In some cases, theprior art methods provide a RDS where the RDS at C₆ is less than that ofC₂ and C₃.

The cellulose esters prepared by the process of the present inventionare precipitated by contacting the cellulose ester solution with anon-solvent to precipitate at least a portion of the cellulose ester,and thereby produce a slurry comprising precipitated cellulose ester andprecipitation liquids. Examples of non-solvents include, but are notlimited to, C₁-C₈ alcohols, water, or a mixture thereof. In oneembodiment, the methanol is utilized for precipitation of the celluloseesters. The amount of non-solvent can be any amount sufficient to causeat least a portion of the cellulose ester to precipitate. In oneembodiment, the amount of non-solvent can be at least about 10 volumes,at least 5 volumes, or at least 0.5 volumes, based on the total volumeof the acylated cellulose solution. The contact time and temperaturerequired for precipitation of the cellulose ester can be any time ortemperature required to achieve the desired level of precipitation. Inembodiments of this invention, the contact time for precipitation isfrom about 1 to about 300 min, from about 10 to about 200 min, or from20 to 100 min. The contact temperature for precipitation can range fromabout 0 to about 120° C., from about 20 to about 100° C., or from 25 toabout 50° C.

Following precipitation, the cellulose esters prepared by the process ofthe present invention can be separated from the cellulose ester slurryto produce a recovered cellulose ester and precipitation liquids. Anysolid/liquid separation technique known in the art for separating atleast a portion of a liquid from slurries of solids can be utilized.Examples of suitable solid/liquid separation techniques suitable for usein the present invention include, but are not limited to,centrifugation, filtration, and the like. In one embodiment, at least 50weight percent, at least 70 weight percent, or at least 90 weightpercent of the precipitation liquids of the cellulose ester slurry canbe removed.

Following separation, the recovered cellulose esters, or wet cake ofcellulose esters, prepared by the process of the present invention canbe washed with a wash liquid using any method known in the art suitablefor washing a wet cake. An example of a washing technique suitable foruse in the present invention includes, but is not limited to, amulti-stage counter-current wash. The wash liquid can be any celluloseester non-solvent. Non-solvents have been previously discussed in thisdisclosure. Examples of suitable non-solvents include, but are notlimited to, C₁-C₈ alcohols, water, or a mixture thereof.

In one embodiment, washing of the recovered cellulose ester can beperformed in such a manner that at least a portion of any undesiredby-products and/or color bodies are removed from the recovered celluloseester to produce a washed cellulose ester. In one embodiment, thenon-solvent utilized as a wash liquid can contain a bleaching agent inthe range of from about 0.001 to about 50 weight percent, or in therange of from 0.01 to 5 weight percent based on the total weight of thewash liquid. Examples of bleaching agents suitable for use in thepresent invention include, but are not limited to, chlorites, such assodium chlorite (NaClO₂); hypohalites, such as NaOCl, NaOBr, and thelike; peroxides, such as hydrogen peroxide and the like; peracids, suchas peracetic acid and the like; metals, such as Fe, Mn, Cu, Cr and thelike; sodium sulfites, such as sodium sulfite (Na₂SO₃), sodiummetabisulfite (Na₂S₂O₅), sodium bisulfite (NaHSO₃) and the like;perborates, such as sodium perborate (NaBO₃.nH₂O where n=1 or 4);chlorine dioxide (ClO₂); oxygen; and ozone. In one embodiment, thebleaching agent employed in the present invention can include hydrogenperoxide, NaOCl, sodium chlorite and/or sodium sulfite. In oneembodiment, at least 70 percent, or at least 90 percent of the totalamount of byproducts and/or color bodies are removed from the recoveredcellulose ester.

Following washing, the washed cellulose esters prepared by the processof the present invention can be dried by any drying methods known in theart to remove at least a portion of the liquid content of the washedcellulose ester product. Examples of drying equipment include, but arenot limited to, rotary dryers, screw-type dryers, paddle dryers, and/orjacketed dryers. In one embodiment, the dried cellulose ester productcomprises less than 5, less than 3, or less than 1 weight percentliquids.

In one embodiment of the invention, following separation of theprecipitated cellulose esters from the cellulose ester slurry preparedby the process of the present invention, at least a portion of thetetraalkylammonium alkylphosphate is recovered from the precipitationliquids described above to produce a recycled tetraalkylammoniumalkylphosphate for possible reuse in cellulose dissolution. In oneembodiment, the one or more alcohols, water, and/or residual carboxylicacids and optionally cosolvents are substantially removed by treatingthe precipitation liquids with at least one liquid/liquid separationprocess. Such separation process can comprise any liquid/liquidseparation process known in the art, such as, for example, flashvaporization and/or distillation. In one embodiment, at least 80, atleast 90, or at least 95 weight percent of the one or more alcohols,water, and/or residual carboxylic acids and optionally cosolvents can beremoved from the precipitation liquids, thereby producing a recycledtetraalkylammonium alkylphosphate.

In another embodiment, when the cellulose ester non-solvent is a C₁-C₈alcohol or a mixture thereof, such as, MeOH, only a portion of alcoholis removed from the precipitation liquids by a liquid/liquid separationprocess to produce a fractionated precipitation liquid. After partialseparation, the total amount of alcohols in the fractionatedprecipitation liquid is in the range of from about 0.1 to about 60weight percent, in the range of from about 5 to about 55 weight percent,or in the range of from 15 to 50 weight percent. The fractionatedprecipitation liquid can then be treated at a temperature, pressure, andtime sufficient to convert the at least a portion of the carboxylic acidcontained in the precipitation liquids to alkyl esters, such as, methylesters, by reacting the carboxylic acids with the alcohol present in thefractionated precipitation liquids. The esterification can be conductedat a temperature in the range of from 100° C. to 180° C., or in therange of from 130° C. to 160° C. Additionally, the pressure duringesterification can be in the range of from about 10 to about 1,000pounds per square inch gauge (“psig”), or in the range of from 100 to300 psig. The fractionated precipitation liquids can have a residencetime during esterification in the range of from about 10 to about 1,000minutes, or in the range of from 120 to 600 minutes. In one embodiment,at least 5, at least 20, or at least 50 mole percent of the carboxylicacids in the precipitation liquids can be esterified during theabove-described esterification to produce a reformed fractionatedprecipitation liquids. Following the above-described esterification, thecarboxylate esters, one or more alcohols, and water produced during theesterification can be substantially removed by treating the reformedfractionated precipitation liquids with at least one liquid/liquidseparation process. It has been found that conversion of the carboxylicacid to an alkyl ester facilitates its removal from the precipitationliquids. In this embodiment, at least 80, at least 90, or at least 95weight percent of the one or more alcohols, carboxylate esters, water,and/or residual carboxylic acids and optionally cosolvents can beremoved from the precipitation mixture, thereby producing a recycledtetraalkylammonium alkylphosphate stream. At least a portion of thecarboxylate esters removed in this process can be converted toanhydrides by CO insertion.

The cellulose esters prepared by the methods of this invention areuseful in a variety of applications. Those skilled in the art willunderstand that the specific application will depend upon the specifictype of cellulose ester as factors such as the type of acyl substituent,DS, MW, and type of cellulose ester copolymer significantly impactcellulose ester physical properties [Prog. Polym. Sci. 2001, 26,1605-1688].

In one embodiment of the invention the cellulose esters are used inthermoplastic applications in which the cellulose ester is used to makefilm or molded objects. Examples of preferred cellulose ester for use inthermoplastic applications include cellulose acetate, cellulosepropionate, cellulose butyrate, cellulose acetate propionate, celluloseacetate butyrate, or a mixture thereof.

In yet another embodiment of the invention, the cellulose esters areused in coating applications. Examples of coating applications includebut, are not limited to, automotive, wood, plastic, or metal coatings.Examples of preferred cellulose esters for use in coating applicationsinclude cellulose acetate, cellulose propionate, cellulose butyrate,cellulose acetate propionate, cellulose acetate butyrate, or a mixturethereof.

In still another embodiment of the invention, the cellulose esters areused in personal care applications. In personal care applicationscellulose esters generally are dissolved or suspended in appropriatesolvents. The cellulose ester can then act as a structuring agent,delivery agent, and film forming agent when applied to skin or hair.Examples of preferred cellulose ester for use in personal applicationsinclude cellulose acetate, cellulose propionate, cellulose butyrate,cellulose acetate propionate, cellulose acetate butyrate, cellulosehexanoate, cellulose 2-ethylhexanate, cellulose laurate, cellulosepalmitate, cellulose stearate, or a mixture thereof.

In still another embodiment of the invention, the cellulose esters areused in drug delivery applications. In drug delivery applications, thecellulose ester can act as a film former such as in the coating oftablets or particles. The cellulose ester can also be used to formamorphous mixtures of poorly soluble drugs thereby improving thesolubility and bioavailability of the drugs. The cellulose esters can beused in controlled drug delivery wherein the drug is released from thecellulose ester matrix in response to external stimuli such as a changein pH. Examples of preferred cellulose ester for use in drug deliveryapplications include cellulose acetate, cellulose propionate, cellulosebutyrate, cellulose acetate propionate, cellulose acetate butyrate,cellulose acetate phthalate, or a mixture thereof.

In still another embodiment of the invention, the cellulose esters areused in applications involving solvent casting of film. Examples ofthese applications include photographic film and protective andcompensation films for liquid crystalline displays. Examples ofpreferred cellulose ester for use in solvent cast film applicationsinclude cellulose triacetate, cellulose acetate, cellulose propionate,and cellulose acetate propionate.

In still another embodiment of the invention, the cellulose esters ofthe present invention can be used in applications involving solventcasting of film. Examples of such applications include photographicfilm, protective film, and compensation film for liquid crystallinedisplays. Examples of cellulose esters suitable for use in solvent castfilm applications include, but are not limited to, cellulose triacetate,cellulose acetate, cellulose propionate, and cellulose acetatepropionate.

In an embodiment of the invention, films are produced comprisingcellulose esters of the present invention and are used as protective andcompensation films for liquid crystalline displays (LCD). These filmscan be prepared by solvent casting as described in US application2009/0096962 or by melt extrusion as described in US application2009/0050842, both of which are incorporated in their entirety in thisinvention to the extent they do not contradict the statements herein.

When used as a protective film, the film is typically laminated toeither side of an oriented, iodinated polyvinyl alcohol (PVOH)polarizing film to protect the PVOH layer from scratching and moisture,while also increasing structural rigidity. When used as compensationfilms (or plates), they can be laminated with the polarizer stack orotherwise included between the polarizer and liquid crystal layers.These compensation films can improve the contrast ratio, wide viewingangle, and color shift performance of the LCD. The reason for thisimportant function is that for a typical set of crossed polarizers usedin an LCD, there is significant light leakage along the diagonals(leading to poor contrast ratio), particularly as the viewing angle isincreased. It is known that various combinations of optical films can beused to correct or “compensate” for this light leakage. Thesecompensation films must have certain well-defined retardation (orbirefringence) values, which vary depending on the type of liquidcrystal cell or mode used because the liquid crystal cell itself willalso impart a certain degree of undesirable optical retardation thatmust be corrected.

Compensation films are commonly quantified in terms of birefringence,which is, in turn, related to the refractive index n. For celluloseesters, the refractive index is approximately 1.46 to 1.50. For anunoriented isotropic material, the refractive index will be the sameregardless of the polarization state of the entering light wave. As thematerial becomes oriented, or otherwise anisotropic, the refractiveindex becomes dependent on material direction. For purposes of thepresent invention, there are three refractive indices of importancedenoted n_(x), n_(y), and n_(z), which correspond to the machinedirection (MD), the transverse direction (TD), and the thicknessdirection, respectively. As the material becomes more anisotropic (e.g.by stretching), the difference between any two refractive indices willincrease. This difference in refractive index is referred to as thebirefringence of the material for that particular combination ofrefractive indices. Because there are many combinations of materialdirections to choose from, there are correspondingly different values ofbirefringence. The two most common birefringence parameters are theplanar birefringence defined as Δ_(e)=n_(x)−n_(y), and the thicknessbirefringence (Δ_(th)) defined as: Δ_(th)=n_(z)−(n_(x)+n_(y))/2. Thebirefringence Δ_(e) is a measure of the relative in-plane orientationbetween the MD and TD and is dimensionless. In contrast, Δ_(th) gives ameasure of the orientation of the thickness direction, relative to theaverage planar orientation.

Optical retardation (R) is related the birefringence by the thickness(d) of the film: R_(e)=Δ_(e)d=(n_(x)−n_(y))_(d);R_(th)=Δ_(th)d=[n_(z)−(n_(x)+n_(y))/2]. Retardation is a direct measureof the relative phase shift between the two orthogonal optical waves andis typically reported in units of nanometers (nm). Note that thedefinition of R_(th) varies with some authors, particularly with regardsto the sign (±).

Compensation films or plates can take many forms depending upon the modein which the LCD display device operates. For example, a C-platecompensation film is isotropic in the x-y plane, and the plate can bepositive (+C) or negative (−C). In the case of +C plates,n_(x)=n_(y)<n_(z). In the case of −C plates, n_(x)=n_(y)>n_(z). Anotherexample is A-plate compensation film which is isotropic in the y-zdirection, and again, the plate can be positive (+A) or negative (−A).In the case of +A plates, n_(x)>n_(y)=n_(z). In the case of −A plates,n_(x)<n_(y)=n_(z).

In general, aliphatic cellulose esters provide values of R_(th) rangingfrom about 0 to about −350 nm at a film thickness of 60 μm. The mostimportant factors that influence the observed R_(th) is type ofsubstituent and the degree of substitution of hydroxyl (DS_(OH)). Filmproduced using cellulose mixed esters with very low DS_(OH) in Shelby etal. (US 2009/0050842) had R_(th) values ranging from about 0 to about−50 nm. By significantly increasing DS_(OH) of the cellulose mixedester, Shelton et al. (US 2009/0096962) demonstrated that largerabsolute values of R_(th) ranging from about −100 to about −350 nm couldbe obtained. Cellulose acetates typically provide R_(th) values rangingfrom about −40 to about −90 nm depending upon DS_(OH).

One aspect of the present invention relates to compensation filmcomprising regioselectively substituted cellulose esters wherein thecompensation film has an Rth range from about −400 to about +100 nm. Inanother embodiment of the invention, compensation films are providedcomprising regioselectively substituted cellulose esters having a totalDS from about 1.5 to about 2.95 of a single acyl substituent (DS≦0.2 ofa second acyl substituent) and wherein the compensation film has an Rthvalue from about −400 to about +100 nm.

In one embodiment of the invention, the regioselectively substitutedcellulose esters utilized for producing films are selected from thegroup consisting of cellulose acetate, cellulose propionate, andcellulose butyrate wherein the regioselectively substituted celluloseester has a total DS from about 1.6 to about 2.9. In another embodimentof the invention, the compensation film has R_(th) values from about−380 to about −110 nm and is comprised of a regioselectively substitutedcellulose propionate having a total DS of about 1.7 to about 2.5. In yetanother embodiment, the compensation film has R_(th) values from about−380 to about −110 nm and is comprised of a regioselectively substitutedcellulose propionate having a total DS of about 1.7 to about 2.5 and aring RDS ratio for C₆/C₃ or C₆/C₂ of at least 1.05. In anotherembodiment, the compensation film has R_(th) values from about −60 toabout +100 nm and is comprised of regioselectively substituted cellulosepropionate having a total DS of about 2.6 to about 2.9. In yet anotherembodiment, the compensation film has R_(th) values from about −60 toabout +100 nm and is comprised of regioselectively substituted cellulosepropionate having a total DS of about 2.6 to about 2.9 and a ring RDSratio for C₆/C₃ or C₆/C₂ of at least 1.05. In another embodiment, thecompensation film has R_(th) values from about 0 to about +100 nm and iscomprised of a regioselectively substituted cellulose propionate havinga total DS of about 2.75 to about 2.9. In yet another embodiment, thecompensation film has R_(th) values from about 0 to about +100 nm and iscomprised of a regioselectively substituted cellulose propionate havinga total DS of about 2.75 to about 2.9 and a ring RDS ratio for C₆/C₃ orC₆/C₂ of at least 1.05.

Another aspect of the present invention relates to compensation filmwith an R_(th) range from about −160 to about +270 nm comprised ofregioselectively substituted cellulose esters having a total DS fromabout 1.5 to about 3.0 of a plurality of 2 or more acyl substituents. Inone embodiment of this invention, the cellulose esters can be selectedfrom the group consisting of cellulose acetate propionate, celluloseacetate butyrate, cellulose benzoate propionate, and cellulose benzoatebutyrate; wherein the regioselectively substituted cellulose ester has atotal DS from about 2.0 to about 3.0. In another embodiment, thecompensation film has R_(th) values from about −160 to about 0 nm and iscomprised of a regioselectively substituted cellulose acetate propionatehaving a total DS of about 2.0 to about 3.0, a ring RDS ratio for C₆/C₃or C₆/C₂ of at least 1.05, and a carbonyl RDS ratio for at least oneacyl substituent for C₆/C₃ or C₆/C₂ of at least about 1.3. In anotherembodiment, the compensation film has R_(th) values from about +100 toabout +270 nm and is comprised of a regioselectively substitutedcellulose benzoate propionate having a total DS of about 2.0 to about3.0, a ring RDS ratio for C₆/C₃ or C₆/C₂ of at least 1.05, and acarbonyl RDS ratio for at least one acyl substituent for C₆/C₃ or C₆/C₂of at least about 1.3. In another embodiment, the compensation film hasR_(th) values from about +100 to about +270 nm and is comprised of aregioselectively substituted cellulose benzoate propionate having atotal DS of about 2.0 to about 2.85, a benzoate DS of about 0.75 toabout 0.90, a ring RDS ratio for C₆/C₃ or C₆/C₂ of at least 1.05, acarbonyl RDS ratio for propionate for C₆/C₃ or C₆/C₂ of at least about1.3.

Further information concerning ionic liquids, their use in theproduction of cellulose esters and cellulose derivatives, the use ofcosolvents with ionic liquids in processes to produce cellulose estersand cellulose derivatives, and treatment of cellulose esters aredisclosed in U.S. patent application entitled “Cellulose Esters andTheir Production In Carboxylated Ionic Liquids” filed on Feb. 13, 2008and having Ser. No. 12/030,387 and its Continuation-In-Part Applicationentitled “Regioselectively Substituted Cellulose Esters Produced In ACarboxylated Ionic Liquid Process and Products Produced Therefrom” filedon Sep. 12, 2009; U.S. patent application entitled “Cellulose Esters andTheir Production in Halogenated Ionic Liquids filed on Aug. 11, 2008 andhaving Ser. No. 12/189,415 and its Continuation-In-Part Applicationentitled “Regioselectively Substituted Cellulose Esters Produced In AHalogenated Ionic Liquid Process and Products Produced Therefrom” filedon Sep. 12, 2009; U.S. patent application “Production of Ionic Liquids”filed on Feb. 13, 2008 having Ser. No. 12/030,425; and U.S. patentapplication entitled “Reformation of Ionic Liquids” filed on Feb. 13,2008 having Ser. No. 12/030,424; U.S. patent application entitled“Treatment of Cellulose Esters” filed on Aug. 11, 2008, having Ser. No.12/189,421; U.S. patent application entitled “Production of CelluloseEsters In the Presence of A Cosolvent” filed on Aug. 11, 2008 havingSer. No. 12/189,753; and U.S. Provisional Application entitled“Regioselectively Substituted Cellulose Esters and Their Production inIonic Liquids” filed on Aug. 13, 2008 having Ser. No. 61/088,423; all ofwhich are incorporated by reference to the extent they do not contradictthe statements herein.

This invention can be further illustrated by the following examples,although it will be understood that these examples are included merelyfor purposes of illustration and are not intended to limit the scope ofthe invention.

EXAMPLES Materials and Methods

Experimental tetraalkylammonium alkylphosphates were prepared asdescribed in the examples. The degree of polymerization of the cellulosewas determined by capillary viscometry using copper ethylenediamine(Cuen) as the solvent. Prior to dissolution, the cellulose was typicallydried for 14-18 h at 50° C. and 5 mm Hg.

The relative degree of substitution (RDS) at C₆, C₃, and C₂ in thecellulose ester of the present invention was determined by carbon 13 NMRfollowing the general methods described in “Cellulose Derivatives”, ACSSymposium Series 688, 1998, T. J. Heinze and W. G. Glasser, Editors,herein incorporated by reference to the extent it does not contradictthe statements herein. Briefly, the carbon 13 NMR data was obtainedusing a JEOL NMR spectrometer operating at 100 MHz or a Bruker NMRspectrometer operating at 125 MHz. The sample concentration was 100mg/mL of DMSO-d₆. Five mg of Cr(OAcAc)₃ per 100 mg of sample were addedas a relaxation agent. The spectra were collected at 80° C. using apulse delay of 1 second. Normally, 15,000 scans were collected in eachexperiment. Conversion of a hydroxyl to an ester results in a downfieldshift of the carbon bearing the hydroxyl and an upfield shift of acarbon gamma to the carbonyl functionality. Hence, the RDS of the C2 andC6 ring carbons were determined by direct integration of the substitutedand unsubstituted C₁ and C₆ carbons. The RDS at C₃ was determined bysubtraction of the sum of the C₆ and C₂ RDS from the total DS. Thecarbonyl RDS was determined by integration of the carbonyl carbons usingthe general assignments described in Macromolecules, 1991, 24,3050-3059, herein incorporated by reference to the extent it does notcontradict the statements herein. In the case of cellulose mixed esterscontaining a plurality of acyl groups, the cellulose ester was firstconverted to a fully substituted cellulose mixed p-nitrobenzoate ester.The position of the p-nitrobenzoate esters indicate the location of thehydroxyls in the cellulose mixed ester.

Color measurements were made following the general protocol of ASTMD1925. Samples for color measurements were prepared by dissolving 1.7 gof cellulose ester in 41.1 g of n-methylpyrrolidone (NMP). A HunterLabColor Quest XE colorimeter with a 20 mm path length cell operating intransmittance mode was used for the measurements. The colorimeter wasinterfaced to a standard computer running Easy Match QC Software(HunterLab). Values (L*; white to black, a*; +red to −green, b*; +yellowto −blue) were obtained for NMP (no cellulose ester) and for thecellulose ester/NMP solutions. Color difference (E*) between the NMPsolvent and the sample solutions were then calculated(E*=[(Δa*)2+[(Δb*)2+[(ΔL*)2]0.5 where Δ is the value for the samplesolutions minus the value for the solvent. As the value for E*approaches zero, the better the color.

Sulfur and phosphorus concentration in cellulose esters were determinedby Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP). Thesamples were prepared by digestion in concentrated HNO₃ followed bydilution with ultra pure water after addition of an internal standard.The final matrix was 5% by weight HNO₃ in water. The samples were thenanalyzed for phosphorus (177.434 nm) and sulfur (180.669 nm) contentusing a Perkin Elmer 2100DV Inductively Coupled Plasma-Optical EmissionSpectrometer that was calibrated using NIST traceable standards.

Solvent casting of film was performed according to the following generalprocedure. Dried cellulose ester and 10 wt % plasticizer were added to a90/10 wt % solvent mixture of CH₂Cl₂/methanol (or ethanol) to give afinal concentration of 5-30 wt % based on cellulose ester+plasticizer toproduce a cellulose ester/solvent mixture. The cellulose ester/solventmixture was sealed, placed on a roller, and mixed for 24 hours to createa uniform cellulose ester solution. After mixing, the cellulose estersolution was cast onto a glass plate using a doctor blade to obtain afilm with the desired thickness. Casting was conducted in a fume hoodwith relative humidity controlled at 50%. After casting, the film andglass were allowed to dry for one hour under a cover pan (to minimizerate of solvent evaporation). After this initial drying, the film waspeeled from the glass and annealed in a forced air oven for 10 minutesat 100° C. After annealing at 100° C., the film was annealed at a highertemperature (120° C.) for another 10 minutes.

Film optical retardation measurements were made using a J.A. WoollamM-2000V Spectroscopic Ellipsometer having a spectral range of 370 to1000 nm. RetMeas (Retardation Measurement) program from J.A. WoollamCo., Inc. was used to obtain optical film in-plane (R_(e)) andout-of-plane (R_(th)) retardations. Values are reported at 589 or 633 nmat a film thickness of 60 μm.

Example 1 Preparation of Tributylmethylammonium Dimethylphosphate[TBMA]DMP

To a 1 L 3-neck round bottom flask equipped for mechanical stirring 400g of freshly distilled tributylamine and 302 g trimethylphosphate (1 eq)were added. This resulted in two clear phases with the top phase beingtributylamine. The flask was then placed in an oil bath, and thereaction mixture was blanked with N₂. The oil bath was heated to 120° C.and the contact mixture was stirred for 49 h resulting in a pale,yellow, homogeneous mixture. After cooling to ambient temperature, thesample solidified giving a white solid (Yield >99%).

Analysis by proton NMR indicated >99% conversion of the startingmaterials to tributylmethylammonium dimethylphosphate. Thermogravimetricanalysis (TGA) indicated the onset of thermal decomposition of the[TBMA]DMP at ca. 240° C. Analysis by differential scanning calorimetry(DSC) showed a melt (Tm) centered at 55° C. during the first heatingscan to 100° C. at 20° C./min. After cooling from the melt at 20° C./minto −100° C. and heating again to 100° C. at 20° C./min, a Tg (glasstransition temperature) was observed at −52° C., two Tc (crystallizationtemperature) were observed at 14 and 23° C., and two Tm were observed at54 and 66° C.

This example illustrates the preparation of a tetraalkylammoniumalkylphosphate having a melting point less than 100° C.

Example 2 Dissolution of Cellulose in TributylmethylammoniumDimethylphosphate ([TMBA]DMP)

Prior to cellulose dissolution, 52.26 g tributylmethylammoniumdimethylphosphate ([TBMA]DMP) was added to a 3-neck 100 mL round bottomflask equipped for mechanical stirring and with a N₂/vacuum inlet. Theflask was placed in an 80° C. oil bath, and the [TBMA]DMP was stirredfor 17 hours at ca. 0.9 mm Hg. The [TBMA]DMP was cooled to 70° C., andan iC10 diamond tipped infrared probe (Mettler-Toledo AutoChem, Inc.,Columbia, Md., USA) was inserted to measure absorbance.

To the [TBMA]DMP was added 7.46 g (12.5 wt %) of cellulose (DP ca. 335)while stirring vigorously (5 min addition). The cellulose easily andquickly dispersed in the [TBMA]DMP to produce a cellulose solution.Vacuum was applied with the aid of a bleed valve (ca. 1 mm Hg), and theoil bath was heated to 100° C. By the time the cellulose and [TBMA]DMPreached 100° C. (ca. 60 min), the mixture was a viscous, translucentcellulose solution with no visible particles. The cellulose solution wasstirred for an additional 65 min at 100° C. at which point it was aclear cellulose solution.

FIG. 1 shows the dissolution of 12.5 wt % cellulose in [TBMA]DMP.Cellulose absorbance was measured at 1000 cm⁻¹. The cellulose was addedto the [TBMA]DMP while heated to 70° C. in order to insure dispersion ofthe cellulose and to avoid clumping. By the time all of the cellulosewas added, the cellulose was beginning to dissolve. The contacttemperature was then increased to 100° C. As the contact temperatureincreased, the rate of cellulose dissolution increased significantly andby the time the cellulose and [TBMA]DMP reached 100° C., the cellulosewas essentially dissolved in the [TBMA]DMP to produce a homogeneouscellulose solution.

This example shows that cellulose can be rapidly and easily dissolved in[TBMA]DMP at significant concentrations.

Example 3 Development of Models for Analysis of the Amount of CelluloseDissolved in Tetraalkylammonium Alkyl Phosphates

Following the general procedure of Example 2, different concentrationsof cellulose (2.5, 5.0, 7.5, 10.0, 12.5 wt %) were dissolved intributylmethylammonium dimethylphosphate. Using initial and finalconcentrations, absorbances at the different concentrations were fitusing a partial least-squares method over the spectral region of860-1320 cm⁻¹. Additionally, a linear least squares fit was made to thesecond derivative of a cellulose absorbance band at 1157 cm⁻¹. This bandwas selected on the basis that there is little or no interference fromtetraalkylammonium alkylphosphates in this spectral region. FIG. 2 showsmodeled wt % cellulose, and the experimental absorbance values for 10 wt% cellulose dissolved in tributylmethylammonium dimethylphosphate. Ascan be seen, the modeled wt % cellulose and the experimental absorbancewere in excellent agreement.

This example illustrates that models based on in situ infraredspectroscopy can be used to determine the concentration of cellulose intetraalkylammonium alkylphosphates. This type of analysis isparticularly useful when the cellulose is only partially soluble at agiven concentration in a particular tetraalkylammonium alkyl phosphate.

Example 4 Comparison of the Dissolution of Cellulose in DifferentTetraalkylammonium Alkylphosphates

The following additional tetraalkylammonium alkylphosphates wereprepared by the general method of Example 1: tributylmethylammoniumdimethylphosphate [TBMA]DMP, tripentyl methylammonium dimethylphosphate[TPMA]DMP, trioctylmethylammonium dimethylphosphate [TOMA]DMP,trimethylethanolammonium dimethylphosphate [TMEA]DMP,trimethylethoxyethanolammonium dimethylphosphate [TMEEA]DMP, andtrimethylethylacetateammonium dimethylphosphate [TMEAA]DMP. Followingthe general method of Example 2 using a fixed concentration of cellulose(7.5 wt %, DP 335), the solubility of cellulose in each of thesetetraalkylammonium alkylphosphates were evaluated. The total amount ofcellulose dissolved was determined using the linear model described inExample 3. The results are summarized in Table 1.

TABLE 1 The amount of cellulose dissolved in selected tetraalkylammoniumalkylphosphates. tetraalkylammonium wt % cellulose % cellulosealkylphosphates dissolved dissolved [TBMA]DMP 7.5 100 [TMEA]DMP 6.5 87[TMEEA]DMP 5.9 79 [TPMA]DMP 2.1 28 [TMEAA]DMP 1.0 13 [TOMA]DMP 0.0 0

This example illustrates that alkyl groups attached to the ammoniumcation with the same alkyl phosphate anion have a very significanteffect on the solubility of cellulose in the ionic liquid. For example,in the case of methylammonium, as the other 3 alkyl groups are changedin the order of C4 (tributyl), C5 (tripentyl), C8 (trioctyl), the weight% cellulose dissolved is 100, 28, and 0 wt %, respectively. Withtrimethylammonium, moving from ethanol to ethoxyethanol diminishescellulose solubility. Upon acetylation of the ethanol, cellulosesolubility is significantly reduced. Clearly, cellulose has excellentsolubility in certain tetraalkylammonium alkylphosphates. Only bycareful experimentation can the utility of a particulartetraalkylammonium alkylphosphates in cellulose dissolution bedetermined.

Example 5 Dissolution of Cellulose in a Mixture ofTributylmethylammonium Dimethylphosphate and Dimethylsulfoxide

To a 3-neck 100 mL round bottom flask equipped for mechanical stirringand having a N₂/vacuum inlet and an iC10 diamond tipped infrared probe(Mettler-Toledo AutoChem, Inc., Columbia, Md., USA) was added a mixtureof 51.31 g of [TBMA]DMP and 17.10 g of dimethyl sulfoxide (DMSO) (25 wt%). While stirring rapidly at room temperature, 7.60 g of cellulose (10wt %, DP ca. 335) was added to the [TBMA]DMP and DMSO solution (6 minaddition). The cellulose/[TBMA]DMP/DMSO mixture was stirred for anadditional 4 min to insure that the cellulose was well dispersed beforeraising a preheated 80° C. oil bath to the flask. Ten minutes afterraising the oil bath, all of the cellulose was dissolved giving a paleyellow, low viscosity, cellulose solution.

This example illustrates that cosolvents may be utilized when dissolvingcellulose in tetraalkylammonium alkylphosphates as a means to lowercellulose solution viscosity.

Example 6 Esterification of Cellulose Dissolved inTributylmethylammonium Dimethylphosphate in the Absence of a Catalyst

A 7.5 wt % cellulose solution was prepared according to the generalmethod of Example 2. The temperature of the cellulose solution wasadjusted to 80° C. prior to adding 3 equivalents of acetic anhydride(Ac₂O) drop wise (6 min) to produce an acylated cellulose solution.Samples of the acylated cellulose solution were removed during thecourse of the reaction, and the cellulose ester was isolated byprecipitation in methanol to produce a cellulose ester slurry. Thecellulose ester slurry was filtered to produce a recovered celluloseester. Each recovered cellulose ester sample was washed with four 200 mLportions of methanol to produce a washed cellulose, then dried at 50°C., 10 mm Hg, to yield a dried cellulose ester product.

FIG. 3 shows a plot of absorbance for infrared bands at 1825 cm⁻¹(acetic anhydride and 1220 cm⁻¹ (acetate ester and acetic acid) versuscontact time during esterification (3 eq acetic anhydride) of cellulosedissolved in [TBMA]DMP. The DS values shown in FIG. 3 were determined byproton NMR spectroscopy and correspond to the acylated cellulosesolution samples removed during the course of the contact period. AsFIG. 3 illustrates, the rate of reaction was very fast. After the startof anhydride addition, only 23 minutes were required to reach a DS of2.52.

This example shows that in the esterification of cellulose dissolved intetraalkylammonium alkylphosphates, it is not necessary to add catalyststo promote the reaction.

Example 7 Esterification of Cellulose Dissolved in1-butyl-3-methylimmidazolium Dimethylphosphate (Comparative)

A 3-neck 100 mL round bottom flask was equipped for mechanical stirring,with an iC10 diamond tipped IR probe (Mettler-Toledo AutoChem, Inc.,Columbia, Md., USA), and with an N2/vacuum inlet. To the flask wereadded 50.65 g of 1-butyl-3-methylimidazolium dimethylphosphate([BMIm]DMP). While stirring at room temperature 4.11 g (7.5 wt %) ofcellulose (DP ca. 335, 6 min addition) were added. Vacuum (2 mm Hg) wasthen applied with the aid of a bleed valve and a preheated 80° C. oilbath was raised to the flask. Six minutes after raising the oil bath, aclear cellulose solution was produced. The cellulose solution wasstirred for an additional 11 min before the oil bath was dropped, andthe cellulose solution was allowed to cool to room temperature.

9.57 g (3.7 eq) of acetic anhydride was added dropwise over the courseof 10 minutes to produce a reaction mixture. After the addition wascomplete, the reaction mixture was stirred for 33 min before a samplewas removed, and the cellulose ester precipitated in methanol (Sample1). At this point, no color had formed in the reaction mixture. Apreheated 50° C. oil bath was raised to the flask. The reaction mixturewas stirred for 34 min before a sample was removed, and cellulose esterprecipitated in methanol (Sample 2). There was little change in thereaction mixture color. The oil bath temperature setting was increasedto 80° C. Within 13 minutes, the reaction mixture color was deep amber,and the viscosity started to increase. After an additional 8 minutes,the reaction mixture gelled. Stirring was continued for an additional 30minutes before dropping the oil bath. The reaction mixture was a blackgel. To solidify the gel, methanol was added directly to the flask(Sample 3). After filtration, each sample was washed with four 200 mLportions of methanol, and the cellulose ester solids were driedovernight at 50° C., 5 mm Hg. Sample 1 (white solid) and Sample 2 (tansolid) were soluble in solvents like DMSO and NMP. Sample 3 (blacksolid) was insoluble in all solvents evaluated. It is important to notethat sample 2 (DS=2.48) was not soluble in acetone at 100 mg/mL; a gelwas formed. In this invention, we have found that cellulose acetatesprepared from cellulose dissolved in tetraalkylammonium alkylphosphatesare fully soluble in acetone at 100 mg/mL when they have a DS from about2.45 to about 2.55.

FIG. 4 shows a plot of absorbance for infrared bands at 1825 cm⁻¹(acetic anhydride and 1220 cm⁻¹ (acetate ester and acetic acid) versuscontact time during esterification of the cellulose dissolved in[BMIm]DMP.

Another reaction was conducted in an identical manner to prior reactionexcept that 0.1 eq MSA was used as a catalyst. As shown in FIG. 5,inclusion of methyl sulfonic acid (MSA) did not change the reaction raterelative to when no MSA was present. As in the case of the reactioninvolving no MSA, after raising the reaction temperature to 80° C., thereaction mixture viscosity was observed to increase. A sample wasquickly removed and precipitated in MeOH. Within a few minutes ofsampling, the reaction mixture also gelled. That is, the presence of MSAdid not inhibit gelation.

These examples illustrate a number of important points. Dissolution ofcellulose in [BMIm]DMP is known (Green Chemistry 2008, 10, 44-46; GreenChemistry 2007, 9, 233-242). However, this example shows thatesterification of cellulose dissolved in [BMIm]DMP is not successful.Esterification of cellulose dissolved in [TBMA]DMP at 80° C. does notlead to gelation (Example 6). The cellulose ester product obtained istypically a white solid having a DS range of 2.45-2.55 and is completelysoluble in acetone. In contrast, esterification of cellulose dissolvedin [BMIm]DMP at 80° C. leads to rapid gelation. The cellulose esterproduct is always highly colored and is insoluble in all solvents.Inclusion of MSA does not change the reaction rates nor does it preventgelation. This is in contrast to what is observed in the esterificationof cellulose dissolved in [BMIm]Cl where inclusion of MSA bothaccelerates the rates of reaction and prevents gelation as disclosed inU.S. patent application entitled “Cellulose Esters and Their Productionin Halogenated Ionic Liquids filed on Aug. 11, 2008 and having Ser. No.12/189,415.

Example 8 Esterification of Cellulose Dissolved in TetraalkylammoniumAlkylphosphates-Strong Acid Mixtures

Prior to cellulose dissolution, to a 3-neck 100 mL round bottom flaskequipped for mechanical stirring and with a N₂/vacuum inlet, 53.46 g oftributylmethylammonium dimethylphosphate ([TBMA]DMP) were added. Theflask was placed in an 80° C. oil bath, and the solution was stirredovernight under vacuum (16.5 h at ca. 0.5 mm Hg). The [TBMA]DMP wascooled to 70° C., and an IR probe (Mettler-Toledo AutoChem, Inc.,Columbia, Md., USA) was inserted to obtain absorbance data.

To the [TBMA]DMP was added 5.94 g (10 wt %) of microcrystallinecellulose while stirring vigorously (5 min addition). The celluloseeasily and quickly dispersed in the [TBMA]DMP. Vacuum was applied withthe aid of a bleed valve (ca. 1 mm Hg), and the oil bath was heated to100° C. By the time the cellulose and [TBMA]DMP reached 98° C. (ca. 20min), nearly all of the cellulose was dissolved. The cellulose and[TMBA]DMP were stirred for an additional 100 min at 100° C. at whichpoint the mixture was a clear, pale yellow, cellulose solution.

To the cellulose solution was added 1.87 g (0.5 eq) of acetic anhydride(Ac₂O) drop wise over the course of 3 min. The cellulose solutionviscosity dropped significantly, and the cellulose solution color becameslightly darker. At this point, the cellulose solution was cooled to 80°C. before adding 9.35 g (2.5 eq) Ac₂O drop wise over the course of 9 minto produce an acylated cellulose solution. Samples of the acylatedcellulose solution were removed at different time intervals, and thecellulose ester was isolated by precipitation in 200 mL methanol toproduce a cellulose ester slurry. Precipitated cellulose ester wasisolated by filtration to produce a recovered cellulose ester. Eachrecovered cellulose ester sample was washed with four 200 mL portions ofmethanol to produce a washed cellulose ester then dried invacuo (50° C.,ca. 10 mm Hg). Based on in situ IR, the reaction was nearly complete 11min after the end of the 2^(nd) anhydride addition. The contact periodwas extended further in order to see how the acylated cellulose solutioncolor changed. As the reaction progressed, the acylated cellulosesolution became darker amber, nearly brown at the end of the reaction.

FIG. 6 shows a plot of absorbance for infrared bands at 1825 cm⁻¹(acetic anhydride) and 1724 cm⁻¹ (acetic acid shifted to a higher wavenumber due to interaction with the [TBMA]DMP) versus contact time duringesterification of cellulose dissolved in [TBMA]DMP. The DS values shownin FIG. 6 were determined by proton NMR spectroscopy and correspond tothe samples removed during the course of the contact period. As FIG. 6illustrates, the rate of reaction at 100° C. was extremely fast. TheAc₂O was consumed so rapidly that Ac₂O was not observed during theaddition. At 80° C., the rate of reaction slowed slightly but was stillvery rapid. After the start of 2^(nd) anhydride addition, only 21minutes were required to reach a DS of 2.48.

After precipitation, washing, and drying, sample 1 (DS=2.04) was a whitesolid. Sample 2 (DS=2.48) was a white solid. Sample 3 (DS=2.61) was anoff-white solid. Sample 4 (DS=2.71) was a pale tan solid. Samples 2(DS=2.48) was fully soluble in acetone at 100 mg/mL.

The precipitation liquids were concentrated in vacuo giving [TBMA]DMPthat contained 8.3 wt % residual acetic acid (HOAc). The cation:anionratio in the [TBMA]DMP was 1:1 indicating no anion exchange.

Two additional reactions were conducted following the exact sameprotocol described above except that 1 wt % perchloric acid (HClO₄)(based on [TBMA]DMP) or 1 wt % methane sulfonic acid (MSA) (based on[TBMA]DMP) was added with the 0.5 eq Ac₂O as a mixture. FIGS. 7 and 8compares a plot of absorbance for infrared bands at 1724 cm⁻¹ versuscontact time during the esterification of cellulose dissolved in[TBMA]DMP when Ac₂O, Ac₂O+HClO₄, or Ac₂O+MSA are added. The DS valuesshown in FIG. 8 were determined by proton NMR spectroscopy andcorrespond to the samples removed during the course of the contactperiod. FIG. 7 shows the contact period involving the addition of 0.5 eqAc₂O at 100° C. The x axis has been shifted so that each reaction beginsat the same point of anhydride addition (15 min). In all 3 cases, therate of reaction at 100° C. was extremely fast. The Ac₂O was consumed sorapidly that Ac₂O was not observed during the addition. FIG. 8 shows thecontact period involving the addition of 2.5 eq Ac₂O at 80° C. The xaxis has been shifted so that each reaction begins at the same point ofanhydride addition (72 min). In all 3 cases, the rate of reaction slowedslightly due to a decrease in temperature but was still very rapid. Thatis, inclusion of these acids did not accelerate the reaction rates. Infact, comparing the initial slopes (72-90 min) of the plots and the DSvalues obtained for each reaction showed that inclusion of MSA slowedthe reaction rates significantly. This is more easily seen by FIG. 9which shows a plot of DS versus time for the time period involvingaddition of 2.5 eq Ac₂O in the absence and presence of acid at 80° C.Furthermore, analysis of these samples by gel permeation chromatography(GPC, Table 2) showed that inclusion of these strong acids at highconcentrations had a negligible impact on the molecular weights of thecellulose ester products. Typically, inclusion of these acids at theseconcentrations would significantly reduce the molecular weights of thecellulose ester products.

TABLE 2 Molecular weights and sample colors for each sample removed fromthe contact mixture during esterification of cellulose dissolved in[TBMA]DMP in the presence and absence of 1 wt % acid. Mn Mw Mz Mw/MnSample Color Ac₂O Sample 1 35171 118384 313653 3.37 white Sample 2 32990119285 304868 3.62 white Sample 3 30601 119971 328116 3.92 off-whiteSample 4 29996 125338 334972 4.18 tan Ac₂O + HClO₄ Sample 1 33266 120082317027 3.61 white Sample 2 31528 124730 325057 3.96 white Sample 3 31788129486 337277 4.07 off-white Sample 4 30956 137136 365184 4.43 tanAc₂O + MSA Sample 1 34895 114799 291538 3.29 white Sample 2 32423 114750288847 3.54 white Sample 3 31805 117425 300574 3.69 white Sample 4 29832121941 320374 4.09 white

Very surprisingly, as noted in Table 2, inclusion of MSA significantlyimproved the color of the reaction solution and of the cellulose esterproduct. In the case of HClO₄ (a strong oxidant), the colors of thecellulose ester products were very similar to that noted above for thereaction involving only Ac₂O (no acid). In the case of MSA, all foursamples removed from the acylated cellulose solution were white. Thecellulose ester of Sample 4 that was isolated from the Ac2O+MSA acylatedcellulose solution was dissolved in NMP, and the cellulose ester/NMPsolution had an E* of 17.8 while the cellulose ester of Sample 4isolated from the Ac2O acylated cellulose solution when dissolved in NMPhad an E* of 31.4. That is, under identical contact conditions,inclusion of MSA in the acylated cellulose solution decreased solutioncolor by ca. 45%.

Sample 2 isolated from the Ac₂O acylated cellulose solution, and sample3 from the Ac₂O+MSA acylated cellulose solution were analyzed forresidual sulfur and phosphorus by ICP. Sample 2 isolated from the Ac₂Oacylated cellulose solution was found to contain 14.8 ppm S and 12.9 ppmphosphorus. Sample 3 from the Ac₂O+MSA acylated cellulose solution wasfound to contain 15.3 ppm S and 24.5 ppm phosphorus. This data indicatesthat cellulose ester product contains little or no sulfate or phosphateinorganic ester.

This example illustrates a number of surprising and important featuresof this invention. When using tetraalkylammonium alkylphosphates as asolvent for cellulose and during subsequent esterification of thecellulose, strong acids can be used to modify the outcome of thereaction. Inclusion of these acids has little or no impact on molecularweights nor do they accelerate reaction rates. Quite surprisingly, atsufficient concentration, the reactions rates are decreased. Inclusionof these acids in the tetraalkylammonium alkylphosphates can change orimprove the color of the products obtained during esterification of thecellulose. Further, it should be noted that the addition of a smallamount of anhydride to the tetraalkylammonium alkylphosphate-cellulosesolutions leads to a significant reduction in solution viscosityallowing a reduction in contact temperatures.

Example 9 Preparation of Cellulose Mixed Esters from Cellulose Dissolvedin Tetraalkylammonium Alkylphosphates-Strong Acid Mixtures

Prior to cellulose dissolution, 61.11 g tributylmethylammoniumdimethylphosphate ([TBMA]DMP) was added to a 3-neck 100 mL round bottomflask equipped for mechanical stirring and with a N₂/vacuum inlet. Theflask was placed in an 80° C. oil bath, and the [TBMA]DMP was stirredovernight under vacuum (ca. 16.5 h at ca. 0.5 mm Hg). The liquid wascooled to 70° C., and an IR probe (Mettler-Toledo AutoChem, Inc.,Columbia, Md., USA) was inserted.

To the [TBMA]DMP was added 6.79 g (10 wt %) of microcrystallinecellulose while stirring vigorously (5 min addition). The celluloseeasily and quickly dispersed in the [TBMA]DMP. The oil bath was thenheated to 100° C. By the time the cellulose and [TBMA]DMP reached 97° C.(ca. 35 min), nearly all of the cellulose was dissolved. The celluloseand [TBMA]DMP was stirred for an additional 75 min at 100° C. at whichpoint the mixture was a clear, pale yellow, cellulose solution.

To the cellulose solution was added a chilled mixture of 2.14 g (0.5 eq)of acetic anhydride (Ac₂O) and 2.73 g propionic anhydride (Pr₂O) dropwise over the course of 3 minutes to produce an acylated cellulosesolution. The acylated cellulose solution viscosity droppedsignificantly, and the acylated cellulose solution color became slightlydarker. At this point, the acylated cellulose solution was cooled to 60°C. before adding a chilled mixture of 4.28 g (1 eq) of Ac₂O and 5.45 gPr₂O drop wise over the course of 7 minutes. Samples of the acylatedcellulose solution were removed at different time intervals, and thecellulose ester was isolated by precipitation in 200 mL of 75/25MeOH/H₂O to produce a cellulose ester slurry. The cellulose ester slurrywas filtered to produce a recovered cellulose ester and precipitationliquids. Each recovered cellulose ester sample was washed with four 200mL portions of 75/25 MeOH/H₂O to produce a washed cellulose ester thendried invacuo (50° C., ca. 10 mm Hg) to produce a dried cellulose esterproduct.

FIG. 10 shows a plot of absorbance for infrared bands at 1815 cm⁻¹(anhydride), 1732 cm⁻¹ (acid shifted to a higher wave number due tointeraction with the [TBMA]DMP), and 1226 cm⁻¹ (ester+acid) versuscontact time during esterification of cellulose dissolved in [TBMA]DMP.The DS values shown in FIG. 10 were determined by proton NMRspectroscopy and correspond to the samples removed during the course ofthe contact period. As FIG. 10 illustrates, the rate of reaction at 100°C. was sufficiently fast that anhydride was not observed during theaddition. At 60° C., the rate of reaction slowed. After the start of the2^(nd) anhydride addition, 142 minutes were required to reach a DS of2.42. Relative to Example 8, the reaction rate after the 2^(nd) additionwas slower primarily due to the difference in contact temperature.

After precipitation, washing, and drying, samples 1 (DS=1.36), 2(DS=1.78), and 3 (DS=2.08) were white solids. Sample 4 (DS=2.42) was apale yellow solid.

An additional reaction was conducted following the exact same protocoldescribed above except 1 wt % methane sulfonic acid (MSA) was added withthe 0.5 eq Ac₂O+0.5 eq Pr₂O as a mixture. FIGS. 11 and 12 compares aplot of absorbance for infrared bands at 1815 cm⁻¹ and 1732 cm⁻¹ versuscontact time during the esterification of cellulose dissolved in[TBMA]DMP when Ac₂O/Pr₂O or Ac₂O/Pr₂O+MSA was added. FIG. 11 shows thecontact period involving the addition of 0.5 eq Ac₂O and 0.5 eq Pr₂O at100° C. The x axis has been shifted so that each reaction begins at thesame point of anhydride addition (30 min). In both cases, the rate ofreaction at 100° C. was fast. In the case of Ac₂O/Pr₂O (no MSA),anhydride was consumed so rapidly that none was observed during theaddition. However, with Ac₂O/Pr₂O+MSA, a small concentration ofanhydride was observed during the addition period. FIG. 12 shows thecontact period involving the addition of 1 eq Ac₂O and 1 eq Pr₂O withand without MSA at 60° C. The x axis has been shifted so that eachreaction begins at the same point of anhydride addition (113 min). Inboth cases, the rate of reaction slowed due to the lower contacttemperature. Based on FIG. 12, it is evident that the rate of anhydrideconsumption was slower when MSA was present in the contact mixture.Moreover, comparing the initial slopes (115-175 min) of the 1732 cm⁻¹absorbances also indicates that inclusion of MSA slowed the reactionrate. This is easily seen in FIG. 14 which shows a plot of DS versustime for the time period involving addition of 1.0 eq Ac₂O and 1.0 eqPr₂O in the absence and presence of acid at 60° C. Furthermore, analysisof these samples by gel permeation chromatography (Table 3) showed thatinclusion of these strong acids at high concentrations had a negligibleimpact on the molecular weights of the products. In esterification ofcellulose using typical solvents, inclusion of these acids at theseconcentrations would significantly reduce the molecular weights of thecellulose ester products.

TABLE 3 Molecular weights and sample colors for each sample removed fromthe contact mixture during esterification of cellulose dissolved in[TBMA]DMP in the presence and absence of 1 wt % MSA. Reaction times weremeasured from the point of 2^(nd) anhydride addition at 60° C. reactiontime Sample (min) Mn Mw Mz Mw/Mn Color Ac₂O/Pr₂O Sample 1 14 51153151020 412998 2.95 white Sample 2 28 52243 143508 387475 2.75 whiteSample 3 51 43906 134878 378363 3.07 white Sample 4 142 34520 131462390056 3.81 pale Ac₂O/Pr₂O + yellow MSA Sample 1 9 53846 140171 3657992.6 white Sample 2 29 49959 132034 353502 2.64 white Sample 3 56 46795129352 344249 2.76 white Sample 4 150 37879 129378 375018 3.42 whiteSample 5 277 33913 129788 390131 3.83 white

Additionally, MSA improved the color of the final cellulose esterproduct relative to when MSA is absent. The cellulose ester of Sample 5that was isolated from the Ac₂O+MSA acylated cellulose solution wasdissolved in NMP, and the cellulose ester/NMP solution had an E* of14.3. The cellulose ester of Sample 4 that was isolated from the Ac₂Oacylated cellulose solution was also dissolved in NMP, and the celluloseester/NMP solution had an E* of 17.8. That is, despite a longer contacttime of 135 min, inclusion of MSA in the acylated cellulose solution hasdecreased color in the acylated cellulose solution relative to when MSAwas absent.

The cellulose ester of Sample 4 isolated from the Ac₂O/Pr₂O acylatedcellulose solution, and the cellulose ester of Sample 5 from theAc₂O/Pr₂O+MSA acylated cellulose solution were analyzed for residualsulfur and phosphorus by ICP. Sample 4 isolated from the Ac₂O/Pr₂Oacylated cellulose solution was found to contain 9.1 ppm S and 121.9 ppmphosphorus. Sample 5 from the Ac₂O/Pr₂O+MSA acylated cellulose solutionwas found to contain 9.4 ppm S and 67.8 ppm phosphorus. These dataindicate that the cellulose ester product contains little or no sulfateor phosphate inorganic ester.

This example illustrates a number of surprising and important featuresof this invention. When using tetraalkylammonium alkylphosphates as asolvent for cellulose and during subsequent esterification of thecellulose, strong acids can be used to modify the outcome of theacylated cellulose solution. Inclusion of these acids has little or noimpact on molecular weights, and they can decrease reaction rates.Inclusion of these acids in the tetraalkylammonium alkylphosphates canchange or improve the color of the products obtained duringesterification of the cellulose. Further, it should be noted that theaddition of a small amount of anhydride to the cellulose solutions leadsto a significant reduction in cellulose solution viscosity allowing areduction in contact temperature. Finally, it should be noted that inpreparing mixed esters as shown in this example, the distribution(regioselectivity) of the acyl groups can be controlled by the order theacylating reagents are added and the concentration at which theacylating reagents are added.

Example 10 Preparation of Cellulose Esters from Cellulose Dissolved inTetraalkylammonium Alkylphosphates-Aprotic Solvent Mixtures

Cellulose (10 wt %, DP ca. 335) was dissolved in a 75/25 mixture byweight of [TBMA]DMP/dimethylformamide according to the general procedureof Example 5 which gave a light yellow solution. The temperature of thecellulose solution was adjusted to 50° C. prior to adding 3 equivalentsof Ac₂O drop wise over the course of 16 minutes to produce an acylatedcellulose solution. After adding the Ac₂O, the acylated cellulosesolution was stirred for 64 min at 50° C. before raising the contacttemperature to 80° C. The color of the acylated cellulose solution waslight yellow throughout the contact period. Samples of the acylatedcellulose solution were removed during the course of the reaction, andthe cellulose ester was isolated by precipitation in methanol to producea cellulose ester slurry. The cellulose ester slurry was filtered toproduce a white, recovered cellulose ester and precipitation liquids.Each sample of the recovered cellulose ester was washed with three 200mL portions of to produce a washed cellulose ester then dried at 50° C.,10 mm Hg to produce a dried cellulose ester product.

FIG. 14 shows a plot of absorbance for infrared bands at 1825 cm⁻¹(acetic anhydride and 1724 cm⁻¹ (acetic acid) versus contact time duringesterification of cellulose dissolved in [TBMA]DMP/DMF. The DS valuesshown in FIG. 14 were determined by proton NMR spectroscopy andcorrespond to the acylated cellulose solution samples removed during thecourse of the contact period. As FIG. 14 illustrates, even at 50° C. therate of reaction was adequate. After the start of anhydride addition,only 23 minutes were required to reach a DS of 0.85. When the contacttemperature was increased to 80° C., reaction rates increased leading toa DS of 2.48 when the reaction was terminated. It is important to notethat the acylated cellulose solution color was maintained throughout thecontact period and that the cellulose esters obtained by sampling theacylated cellulose solution were white.

A second reaction was conducted in an identical manner using thecellulose solution prepared in Example 5 (10 wt % cellulose in 75/25[TBMA]DMP/DMSO). In this case, even at 50° C., the acylated cellulosesolution color rapidly darkened, and the acylated cellulose solutionviscosity decreased significantly. When the contact temperature wasincreased to 80° C., the acylated cellulose solution color turned black,and the solution viscosity became extremely low. The cellulose esterswere isolated by precipitating with methanol to produce a celluloseester slurry followed by filtration to produce a recovered celluloseester. The cellulose esters of Samples 1 and 2 were white; Sample 3 wasdeep brown; and Sample 4 was gray-black in appearance. Table 4 comparesthe molecular weights of the cellulose ester products obtained from the[TBMA]DMP/DMSO acylated cellulose solution to the cellulose esterproducts from the [TBMA]DMP/DMF acylated cellulose solution (Table 4).In the case of the [TBMA]DMP/DMSO acylated cellulose solution, both Mnand Mw for the cellulose acetates were significantly lower relative tothe cellulose acetates from the [TBMA]DMP/DMF acylated cellulosesolution. Without wishing to be bound by theory, the discoloration andcellulose ester product degradation is believed to be arise from therelatively acidic proton alpha to the SO double bond which is absent inthe case of DMF.

TABLE 4 Molecular weights for each sample removed from the acylatedcellulose solution during esterification of cellulose dissolved in[TBMA]DMP/DMSO or [TBMA]DMP/DMF. Reaction times are from the end ofanhydride addition. reaction time (min) DS Mn Mw Mz Mw/Mn [TBMA]DMP/DMSO Sample 1 6 0.73 19441 75483 272860 3.88 Sample 2 60 1.68 8531 28050146489 3.29 Sample 3 86 2.12 8413 41964 383794 4.99 Sample 4 113 2.237717 43316 389919 5.61 [TBMA]DMP/ DMF Sample 1 7 0.85 25365 120596499556 4.75 Sample 2 60 1.80 23833 126008 502791 5.29 Sample 3 91 2.2421452 128833 513726 6.01 Sample 4 128 2.48 18857 134760 553723 7.15

This example shows that although a variety of aprotic solvents can beused as co-solvents with tetraalkylammonium alkylphosphates to dissolvecellulose, only selected aprotic solvents, e.g. N,N-dimethylformamide orN-methylpyrrolidone, are appropriate co-solvents during celluloseesterification. In general, any aprotic solvent having a dielectricconstant greater than about 30 that does not bear an acidic protonleading to side reactions is suitable for use in cellulose dissolutionand esterification when used in conjunction with tetraalkylammoniumalkylphosphates.

Example 11 Preparation of Cellulose Acetate Propionate from CelluloseDissolved in 75/25 TributylmethylammoniumDimethylphosphate/N-Methylpyrrolidone

Cellulose (10 wt %, DP ca. 335) was dissolved in a 75/25 mixture byweight of [TBMA]DMP/NMP at 100° C. in 10 minutes according to thegeneral procedure of Example 5 which gave a light yellow cellulosesolution. To the cellulose solution at 100° C. was added 3.3 eq Pr₂O(contained trace amount of Ac₂O) drop wise over the course of 13 minutesto produce an acylated cellulose solution. Fifteen minutes from the endof addition, the IR absorbance values begin to plateau, and theabsorbance values indicated that the DS was near the desired value. TheIR probe was removed from the acylated cellulose solution, and theacylated cellulose solution was immediately poured into 300 mL of 75/25Methanol/H₂O while mixing with a Heidolph homogenizer to produce acellulose ester slurry comprising precipitated cellulose ester andprecipitation liquids. The precipitated cellulose ester was isolated byfiltration than washed 3× with 200 mL portions of 75/25 MeOH/H₂O toproduce a washed cellulose ester before drying overnight at 10 mm Hg,50° C. which provided 11.9 g of a snow white, dried cellulose estersolid. Analysis by ¹H NMR revealed that the cellulose ester had a DS of2.34 (DS_(Pr)=2.30, DS_(Ac)=0.04). Analysis by quantitative carbon 13NMR showed that the dried cellulose ester product was regioselectivelysubstituted having a ring RDS of: RDS C₆=0.97, RDS C₃=0.64, RDS C₂=0.73.The cellulose ester product was fully soluble in a variety of solventsincluding DMSO, NMP, acetone, and 90/10 CH₂Cl₂/MeOH at 100 mg/1 mL.

This example illustrates that cellulose can be rapidly dissolved intetraalkylammonium-aprotic solvents mixtures then esterified at elevatedtemperatures to obtain high quality cellulose esters.

Example 12 Casting of Film and Film Optical Measurements

A series of essentially 6,3- and 6,2-regioselectively substitutedcellulose esters (1-3) were prepared according to the general procedureof Example 11 (high C₆ RDS). Commercial (Comparative Examples 4 and 6)cellulose esters available from Eastman Chemical Company, were producedby the general procedures described in U.S. Patent Publication2009/0096962 and U.S. Patent Publication 2009/0050842. ComparativeExample 5 was prepared as described in U.S. Patent Publication2005/0192434. The cellulose ester in Example 5 is essentially2,3-regioselectively substituted and differed from the examples of thepresent invention in that it has a low RDS at C₆ while the celluloseesters of the present invention have a high RDS at C₆. The ring RDS wasdetermined for each sample before film was cast, and the film opticalproperties determined. The results are summarized in Table 5.

TABLE 5 The degree of substitution, relative degree of substitution, andout-of- plane retardation (nm) for compensation film for celluloseesters of the present invention versus comparative (C) cellulose esters.Example DS DS_(Pr) DS_(Ac) RDS C₆ RDS C₃ RDS C₂ R_(th) (589) 1 2.49 2.440.05 1.00 0.70 0.78 −118.7 2 2.45 2.44 0.01 1.00 0.68 0.76 −140.4 3 2.102.08 0.02 0.95 0.60 0.55 −381.9 4 (C) 2.73 2.69 0.04 0.83 0.98 0.90−29.2 5 (C) 1.99 1.94 0.05 0.36 0.80 0.83 −80.2 6 (C) 1.93 1.77 0.160.56 0.71 0.66 −209.9

Comparing the values of R_(th) for the 6,3-, 6,2-cellulose propionate(Example 3, DS=2.10 C₆/C₃=1.58, C₆/C₂=1.73, DS*C₆/C₃=3.3, DS*C₆/C₂=3.6)to the 2,3-cellulose propionate (Example 5, DS=1.99 C₆/C₃=0.45,C₆/C₂=0.43, DS*C₆/C₃=0.90, DS*C₆/C₂=0.86), it was evident that the 6,3-,6,2-cellulose propionate provided a much more negative R_(th) value(−382 nm versus −80 nm) even though the two cellulose esters had similarDS values. Similarly, the R_(th) value was significantly more negativefor Example 3 relative to Example 6 (DS=1.93 C₆/C₃=0.79, C₆/C₂=0.85,DS*C₆/C₃=1.5, DS*C₆/C₂=1.6). When the total DS of the regioselectivelysubstituted 6,3-, 6,2-cellulose propionate was increased (Examples 1 and2), the values of R_(th) increased rapidly and significantly. Forexample, at a DS of 2.45 (Example 2), R_(th) increased to −140 nm. Ifthe DS is increased slightly to DS 2.49 (Example 2), R_(th) increasedfurther to −119 nm. As FIG. 15 illustrates, if the total DS wasincreased further, R_(th) became positive. This behavior wassignificantly different relative to that observed with commercialcellulose esters (cf. Example 4).

This example illustrates that the substitution pattern of the celluloseester can significantly impact out-of-plane retardation. Specifically,the regioselectively substituted cellulose esters of the presentinvention can provide retardation values not accessible using celluloseesters with different substitution patterns. At lower total DS, R_(th)for the cellulose propionates of the present invention was much morenegative relative to conventional cellulose propionates. At a highertotal DS, R_(th) for the cellulose propionates of the present inventionare less negative or more positive relative to conventional cellulosepropionates.

Example 13 Preparation of Cellulose Acetate Propionate Benzoate from aRegioselectively Substituted Cellulose Acetate Propionate

The cellulose acetate propionate prepared in Example 11 (5 g) andpyridine (50 g) was charged to a 300 mL, round-bottom flask equippedwith a mechanical stirrer and water condenser. The mixture was stirredat room temperature under nitrogen atmosphere to yield a clear solution.Benzoyl chloride (12.8 g) was then added drop wise through an additionfunnel. After the completion of addition (ca. 1 h), the temperature wasraised to 70° C. The mixture was stirred for additional 5 h and thenallowed to cool to room temperature. The resulting solution wasprecipitated into methanol (800 g) with vigorous stirring, filtered,washed repeatedly with methanol, and dried under vacuum to yield afibrous product (4.7 g). Analysis by ¹H NMR revealed a DS_(Pr)=2.29,DS_(Ac)=0.04, DS_(Bz)=0.70. Analysis by ¹³C NMR revealed that theregioselectivity of the initial sample was preserved under the reactionconditions.

Following the general procedure for film casting and for measuring filmoptical properties, the cellulose acetate propionate benzoate was foundto have an out-of-plane retardation (R_(th), 633) of +156 nm at a filmthickness of 43 μm (+218 at 60 μm).

This example showed that regioselective placement of aliphatic acylgroups led to cellulose esters with high C₆/C₃ or C₆/C₂ ratios followedby subsequent placement of an aromatic group at C2 and C3 providedcellulose esters having large, positive values of R_(th). This exampleshowed that these types cellulose esters can be prepared by a 2-stepprocess without perturbing the substitution pattern established in thefirst step.

Example 14 Preparation of Cellulose Benzoate Propionate from CelluloseDissolved in 70/30 TributylmethylammoniumDimethylphosphate/N-Methylpyrrolidone Using a Staged Anhydride Addition

Cellulose (10 wt %, DP ca. 335) was dissolved in a 70/30 mixture byweight of [TBMA]DMP/NMP at 100° C. according to the general procedure ofExample 5 which gave a light yellow cellulose solution. To the cellulosesolution at 100° C. was added 2.5 eq Pr₂O drop wise over the course of 3minutes, to produce an acylated cellulose solution. Ten minutes afterthe end of Pr₂O addition, benzoic anhydride (4 eq) was added in oneportion as a melt (melted at 85° C.). The acylated cellulose solutionwas stirred for an additional 35 minutes at which time the IR absorbancevalues indicated that the DS was near the desired value. The IR probewas removed from the acylated cellulose solution, and the acylatedcellulose solution was immediately poured into 300 mL of methanol whilemixing with a Heidolph homogenizer to produce a cellulose benzoatepropionate slurry. The cellulose benzoate propionate was isolated byfiltration then washed 8× with 200 mL portions of methanol before dryingovernight at 10 mm Hg, 50° C. Analysis by ¹H NMR revealed that thecellulose benzoate propionate had a DS of 2.91 (DS_(Pr)=2.58,DS_(Bz)=0.33). Analysis by quantitative carbon 13 NMR showed that theproduct was regioselectively substituted having a ring RDS of: RDSC₆=1.00, RDS C₃=0.91, RDS C₂=1.00 and a benzoate carbonyl RDS of: RDSC₆=0.04, RDS C₃=0.12, RDS C₂=0.17. The product was soluble in a varietyof solvents including DMSO, NMP, and CH₂Cl₂.

Following the general procedure for film casting and for measuring filmoptical properties, the cellulose benzoate propionate was found to havean out-of-plane retardation (R_(th), 589) of +50.5 nm at a filmthickness of 27 μm (+112.2 at 60 μm).

This example showed that by employing a staged addition in which thealiphatic reagent is added first followed by addition of the aromaticreagent led to regioselective placement of aliphatic acyl groups (highC₆/C₃ or C₆/C₂ ratios) and installation of the aromatic group at C₂ andC₃. Compensation film prepared from these regioselective substitutedcellulose esters exhibited large, positive values of R_(th).

That which is claimed is:
 1. A process for producing cellulose esters,said process comprising: a) contacting cellulose with at least onetetraalkylammonium alkylphosphate to form a cellulose solution, whereinsaid tetraalkylammonium alkylphosphate is represented by the followingformula

wherein R₁, R₂, R₃, and R₄ are independently selected from the groupconsisting of C₁-C₅ straight chain alkyl groups, branched alkyl groups,and C₂-C₂₀ alkoxy groups; and R₅ and R₆ are independently selected fromthe group consisting of hydrido, C₁-C₅ alkyl groups, and C₂-C₂₀ alkoxygroups; b) contacting said cellulose solution with at least oneacylating reagent selected from the group consisting of aceticanhydride, propionic anhydride, butyric anhydride, isobutyric,anhydride, valeric anhydride, hexanoic anhydride, 2-ethylhexanoicanhydride, nonanoic anhydride, lauric anhydride, palmitic anhydride,stearic anhydride, benzoic anhydride, substituted benzoic anhydrides,phthalic anhydride, and isophthalic anhydride to produce an acylatedcellulose solution comprising at least one cellulose ester; and c)separating at least a portion of said cellulose ester from said acylatedcellulose solution to produce a recovered cellulose ester.
 2. Theprocess according to claim 1, wherein R₁ is methyl or ethyl; R₂, R₃ andR₄ are different and independently selected from the group consisting ofmethyl, ethyl, propyl, butyl, isobutyl, pentyl, ethanol, andethoxyethanol; and R⁵ and R⁶ are independently selected from the groupconsisting of methyl, ethyl, propyl, and butyl.
 3. The process accordingto claim 2, wherein R₁ is methyl; R₂, R₃, and R₄ each are propyl orbutyl; and R₅ and R₆ each are methyl or ethyl.
 4. The process accordingto claim 1, wherein said tetraalkylammonium alkylphosphate comprisestributylmethylammonium dimethylphosphate, tributylethylammoniumdiethylphosphate, tripropylmethylammonium dimethylphosphate,tripropylethylammonium diethylphosphate, or mixtures thereof.
 5. Theprocess according to claim 1, further comprising separating saidtetraalkylammonium alkylphosphate from said acylated cellulose solutionto produce a recovered tetraalkylammonium alkylphosphate.
 6. The processaccording to claim 1, wherein step (c) further comprises contacting saidacylated cellulose solution with at least one non-solvent to cause saidcellulose ester to precipitate to produce a cellulose ester slurrycomprising precipitated cellulose ester and said tetraalkylammoniumalkylphosphate.
 7. The process according to claim 6, further comprisingseparating at least a portion of said precipitated cellulose ester fromsaid cellulose ester slurry to produce said recovered cellulose esterand precipitation comprising said tetraalkylammonium alkylphosphate. 8.The process according to claim 7 further comprising removing at leastone compound selected from the group consisting of alcohols, water,residual carboxylic acids, and cosolvents from said precipitationliquids by at least one liquid/liquid separation process to produce saidrecovered tetraalkylammonium alkylphosphate.
 9. The process according toclaim 6, wherein said non-solvent comprises a C₁-C₈ alcohol, water, or amixture thereof.
 10. The process according to claim 6 wherein the amountof non-solvent is at 0.5 volumes based on the total volume of saidacylated cellulose solution.
 11. The process according to claim 6wherein the amount of non-solvent is at least 10 volumes based on thetotal volume of said acylated cellulose solution.
 12. The processaccording to claim 1, wherein said acylating reagent is added to saidcellulose solution in one or more stages.
 13. The process according toclaim 1 further comprising washing said recovered cellulose ester with awash liquid to produce a washed cellulose ester.
 14. The processaccording to claim 13 wherein said wash liquid comprises a non-solventand, optionally, a bleaching agent.
 15. The process according to claim14 wherein said wash liquid comprises said bleaching agent, wherein saidbleaching agent comprises about 0.001 to about 50 weight percent basedon the total weight of the wash liquid of at least one compound selectedfrom the group consisting of chlorites, hypohalites, peroxides,peracids, metals, sodium sulfites, perborates, chlorine dioxide, oxygen,and ozone.
 16. The process according to claim 1, wherein said recoveredcellulose ester comprises cellulose acetate, cellulose propionate,cellulose butyrate, cellulose acetate propionate, cellulose acetatebutyrate, or a mixture thereof.